How often should Birchas Hatorah be recited?
The blessing over Torah study is recited only once a day, in the morning [as part of Birchas Hashachar], prior to beginning learning Torah that day. It is not recited again any other time that day, until the next morning. This applies even if one stopped learning and did not have plans to continue learning any more that day, and then decided to learn again. Likewise it applies even if one slept during the day and then decided to continue learning after awakening.
 47/7 as rules the second opinion there and as concludes Admur that so is the custom; So also plainly rules Siddur as is evident from the fact Admur rules in Siddur that one may say a blessing even if he did not sleep at night, hence completely accepting the custom and second opinion; So rules also Michaber 47/11; Ashel Avraham Butchach; Kaf Hachaim 47/25; Toras Chaim Sofer 11; Minchas Shlomo 18
In 47/7 Admur records a dispute regarding whether one must repeat the blessing of Birchas Hatorah if he made an interval prior to learning a second time that day. The basis of this dispute is with regards to whether we view Birchas Hatorah similar to Birchas Hashachar or not.
First Opinion: There are opinions [Mahram; Rosh] which rule that the blessing said over Torah study is not like the morning blessings and thus one must say a new blessing if he completely removed his mind from his previous learning session. This means as follows: According to all if one began learning after Birchas Hatorah and then stopped learning in order to work and then resumed learning, he is not required to repeat the blessing, if he is a person that is accustomed to constantly learn. As such a person tries to finish his work as soon as possible in order so he can resume learning. Thus this work is not considered an interval between the blessing [and the resumed learning] as even while he was working he was thinking about his learning. The same applies if he went to the bathhouse or bathroom, it is not considered an interval, as even while he is in the bathhouse and bathroom he does not remove his mind from resuming his learning afterwards. Likewise if he attempted to sleep and was unable to fall asleep it is not considered an interval being that so long as he is still awake his mind is on his learning. Likewise if he took a mere nap on his arm it is not considered an interval. If however he slept a regular sleep on his bed it is considered an interval [and the blessing must be repeated]. Likewise one who is not accustomed to constantly learn Torah, and said the blessing and learned his daily learning and then went to work, removing his mind from learning any more that day, then if he changes his mind and decides to learn again that day, he must repeat the blessing.
Second Opinion: There are opinions [Igur] who rule that the blessing over Torah study is just like Birchas Hashachar which the Sages instituted to be said only once a day. Therefore even if one slept on his bed, or stopped learning and did not have plans to continue learning any more that day, and then decided to learn more, he is not required to repeat the blessing.
Final Ruling of Admur: The custom is like the second opinion.
Other Opinions: The following Poskim rule like the first opinion that the blessing is to be repeated if an interval took place, such as sleeping during the day: Lechem Chamudos; Peri Chadash; Elya Raba; Gr”a; Chayeh Adam [brought in M”B 47/25]; M”B ibid concludes that one may rely on these Poskim to repeat the blessing. Chesed Lealafim 47/4; and Ben Ish Chaiy Vayeishev 12 rule one is to think the blessing in his mind or recite it without Hashem’s name. Kaf Hachaim 47/25 concludes one is to intend during the blessing to fulfill all the learning of that day and possibly this is then valid according to all.
 According to all, the learning that one does at night prior to having slept is included in the mornings blessing, as the night follows the day in these matters. [ibid; See Piskeiy Teshuvos 47/15]