How often is one to wear Tefillin?
The Mitzvah of Tefillin is for one to wear the Tefillin throughout the entire day. [See Q&A regarding if this is a Biblical obligation] The custom however [in today’s times] is not to wear it the entire day. Rather, the Tefillin is worn only during the Shacharis prayer, as explained next. [One is Biblically obligated to wear Tefillin at least one time every single weekday and one who goes a day without wearing Tefillin nullifies the Biblical positive command. Thus, if one did not wear Tefillin by Shacharis, then he must wear it later on in the day.]
How long must the Tefillin remain on the person? There is no minimum time that the Tefillin must remain on the person in order to fulfill the Mitzvah. Thus, even if the person wears it only for a mere moment, the Mitzvah is fulfilled. [This applies even if one wore the Tefillin without saying anything. Nevertheless, it is [Rabbinically] obligatory to recite the Parshiyos of Shema and Vehaya Im Shamoa, and according to some Poskim also the Parshiyos of Kadeish and Vehaya Ki Yeviacha, while wearing Tefillin as will be explained in the next Halacha!]
Fulfills Mitzvah at every moment: At every moment that one wears Tefillin he is fulfilling the positive command.
How often is one Biblically required to wear Tefillin?
It is unclear from the Talmud and Rishonim as to the essence of the command of wearing Tefillin and as to its exact intent. There are three possibilities:
1. One is obligated to wear it constantly.
2. One is obligated to wear it one time a day.
3. One is obligated to wear it whenever he can, not necessarily once a day.
Some Poskim rule that the Biblical command is to wear Tefillin constantly, the entire day and night, and one who does not wear it constantly nullifies the Biblical command. Other Poskim rule the Biblical obligation is to wear the Tefillin only once a day, although one who wears Tefillin more than once a day fulfills the Biblical command each time he wears it. Nevertheless, even according to this opinion it is a Mitzvah Min Hamuvchar to wear it constantly throughout the day, and thus constantly fulfill the Biblical command. Other Poskim suggest that although the Biblical obligation is to wear the Tefillin only once a day perhaps it is a Rabbinical obligation to wear the Tefillin throughout the day. [See footnote for opinion of Admur]
May one be stringent to wear the Tefillin throughout the day?
No. Even if one knows that he has a clean body he is not to wear the Tefillin throughout the day.
When did people stop wearing Tefillin the entire day?
The Talmud Yerushalmi states that people stopped wearing Tefillin due to a certain incident involving a person who wore Tefillin all day and ended up stealing money entrusted to him. The money was entrusted to him due to his pious outward appearance, as the person who gave him the money found him late Shabbos afternoon wearing Tefillin and Davening. Due to this incident people stopped wearing Tefillin in order not to give room to the Reshaim to make their external appearance be like a Tzaddik. The Rosh explains that the Gemara does not mean to say that they completely stopped wearing Tefillin, but that they stopped wearing Tefillin all day as they are supposed to. From this explanation it is evident that people stopped wearing Tefillin all day already in the times of the Jerusalem Talmud [200 C.E.], and possibly beforehand [depending on the time of the above incident]. It is also understood from this story that not all people wore Tefillin all day, and only the extra pious did so.
Did the Kohanim in the Temple wear Tefillin?
The hand Tefillin is considered a Chatzitza during the Avoda doen in the Mikdash. Therefore it was forbidden for the Kohen to wear his hand Tefillin while serving in the Temple.
 Admur 37/2; Michaber 37/2
 Admur ibid; Michaber ibid; Levush 37/2; See also Admur 25/32; 38/1 and 9 and 11; 39/7 which implies that it is a [Biblical] obligation to wear the Tefillin the entire day.
 Admur ibid; 25/32; Michaber ibid
The reason: Being that Tefillin require a clean body, that one not release flatulence in them, and that he not remove his mind from them, as explained in 28/1, therefore we are accustomed not to wear the Tefillin throughout the entire day, as not every person is able to be careful in these matters. Nevertheless every person must be careful in them to wear them during Kerias Shema and Tefila, (as by these times one anyways must guard his body from flatulence and Hesech Hadaas, as explained in 28/1). [Admur ibid; Michaber ibid] The inner reason behind this is because today, due to the descent of the generations, we are no longer able to draw down any more G-dliness through wearing the Tefillin the entire day. [Toras Menachem 1952 Shelach p. 19]
 Levush 25 [unlike Levush 37/2 that requires to wear entire day]; Shiltei Giborim in Magen Giborim 37/2; Yeshuos Yaakov 37; Minchas Chinuch 378; P”M 37 A”A 2 and P”M 37 A”A 1 in name of Nachals Tzevi 1 and P”M 37 A”A 7; M”B 37/2 that one who misses a day of Tefillin is called a Poshei Yisrael; Kaf Hachaim 30/18; See Piskeiy Teshuvos 25 footnote 1
Other opinions: According to those that rule that Tefillin is a constant Biblical command [see Q&A] there is room to learn that in today’s times that we no longer wear Tefillin throughout the day, there is no reason to require one to wear Tefillin specifically once a day.
 Levush 37/2; see Piskeiy Teshuvos 37/1
 Admur 592/7; 38/7 “Even though he is fulfilling a Mitzvah…”
 See P”M 37 A”A 2
 Possible way of learning Rambam Tefillin 4/25, brought in Magen Giborim 37/2 in name of Achronim; Levush 37/2 [There he says that Ukeshartem implies constant wearing, and so learns Os Chaim Veshalom 25/6. However see Levush 25 and continuation of 37/2 that wearing it once a day even momentarily also suffices]; Suggestion in P”M 37 A”A 2; Sheilasos Bo 45 that one who does not wear it the entire day “does not complete his obligation”
 See Moadim Uzmanim 3/157 that according to those Rishonim that the Mitzvah of wearing Tefillin applies also at night, then the Mitzvah is constant by day and night and not just during the day.
 The reason: As the verse states “A sign on your hands” and a sign is needed constantly. [Levush ibid] This is also the simple implication of the Gemara in Eiruvin 96a and Menachos 36b that the obligation is constant and only gets a break on Shabbos and Yom Tov. [Yalkut Yosef Tefillin, introduction] However perhaps one can say in the Gemara that the obligation is not constant, and the Gemara is teaching us that it is forbidden to wear it on Shabbos and Yom Tov. Or perhaps the Gemara does not intend to say one must wear it the entire day but only once a day.
 Shiltei Giborim in Magen Giborim 37/2 in Rambam Tefillin 4/25; Yeshuos Yaakov 37; Minchas Chinuch 378; Suggestion of P”M 37 A”A 2; Implication of Rosh in Menachos Tefillin 28: The Rosh there explains based on the Talmud Yerushalmi that people stopped wearing Tefillin due to a certain incident involving a person who wore Tefillin all day and ended up stealing money entrusted to him. He also implies that even before this incident not all people wore Tefillin all day, and only the extra pious did so. This implies that according to the Rosh the entire Mitzvah of wearing Tefillin all day is a Hiddur and not an obligation. See Toras Menachem 28th Sivan 1952;
 Magen Giborim ibid; P”M ibid
 Suggestion in P”M 37 A”A 2; Poskim in Os Chaim Veshalom 25/6
 Opinion of Admur: It is implied from Admur that it is an obligation to wear the Tefillin the entire day, although it remains unclear as to whether it is Biblcial or Rabbinical. [In Admur 37/2 he writes “Their Mitzvah is to be on him the entire day” which implies that the actual Biblical command is to wear it constantly. So is also implied from Admur 38/9 and 39/7 in which Admur states “he is exempt from wearing Tefillin the entire day” which implies that normally a person is obligated to do so and it is not just a proper action or act of piety. Nevertheless, perhaps the term Mitzvah and exempt here refers to a Rabbinical obligation and not Biblical, as questions the P”M ibid Vetzaruch Iyun!
 M”A 37/2; Rosh Tefillin 28
 The reason: As this will cause others [i.e. Ramaim] who cannot keep a clean body to also do so. [M”A ibid]
 Brachos 2/3
 Rosh Menachos Hilchos Tefillin 28
 Rambam Keli Hamikdash 6; Zevachim 19a