When during the day is one to wear Tefillin:
A. How often during the day?
The Mitzvah of Tefillin is for one to wear the Tefillin throughout the entire day. The custom however [in today’s times] is not to do so. Rather the Tefillin is worn only during the Shacharis prayer, as explained next.
B. Davening with Tefillin:
 37/2; Michaber 37/2
 Admur ibid; See also 38/1 and 9 and 11; 39/7
 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] The inner reason behind this is because today, due to the descent of the generations, we are no longer able to draw down any extra Elokus through wearing the Tefillin the entire day. [Toras Menachem 1952 Shelach p. 19]
 25/1; 37/2; Michaber 25/1 and 37/2
 The reason why one must wear it specifically by Shema and Shemoneh Esrei: a) As any person who says Shema without Tefillin is considered as if he is testifying falsely of himself, as he recites “Ukeshartem Leos” and he is not doing so. [Admur 25/1] b) Ideally one is obligated to wear Tefillin throughout the day and it is only due to inability to guard the body that we no longer do so, and hence every person must be careful 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) [37/2]
 In 25/1 Admur records “immediately after”, as writes the Tur and is explained in the Bach. However in the Siddur Admur omits the word “immediately” as does the Michaber 25/1.