The time to recite Keriat Shema lasts for the first three hours of the day, for until then people are still waking up. In that way, one indeed fulfills the mitzvah of reciting Shema “u’vekumecha,” (“when you get up”) – when people wake from their sleep. Most poskim favor the opinion of the Gra. However, regarding Keriat Shema, whose time is biblical, it is correct to be stringent like the Magen Avraham, in accordance with the rule, “sefeika d’oraita l’chumra” (“When there is uncertainty concerning biblical commandments we are stringent.”) Nevertheless, there are those who are accustomed l’chatchilah to follow the Gra’s approach, and one should not oppose this custom.
Even though the time of Keriat Shema only lasts three hours, if the third hour passed and one did not recite it, he must recite it with its blessings sometime within the fourth hour, and he is merited for reciting it like one who reads from the Torah (Shulchan Aruch 58:6).
The time of the Shacharit prayer and Birkot Keriat Shema lasts for four hours, for the Chachamim established its time corresponding to the Tamid offering which was brought until the end of the fourth hour of the day. Since its obligation is rabbinic, the halachah is lenient and allows us to rely upon the Gra’s approach and to pray later. Even so, l’chatchilah, those who are expeditious fulfill their obligations early. Therefore, it is good to follow the approach of the Magen Avraham and pray early (Mishnah Berurah 58:4).
If four hours passed and one did not yet recite Birkot Keriat Shema nor pray the Amidah, it is agreed that he may pray the Amidah until chatzot. Although the halachah follows Rabbi Yehudah who rules that the time of Shacharit is until the end of four hours, and does not follow the opinion of the Chachamim who maintain that the time is until chatzot, nevertheless, even Rabbi Yehudah admits that one who did not pray before four hours lapsed may, b’dieved, complete his prayer until chatzot. Although he is not merited for praying on time, he does receive merit for his prayer. One may not pray Shacharit after chatzot (Berachot 27a; Shulchan Aruch Orach Chaim 89:1).
Regarding Birkot Keriat Shema, the poskim disagree. According to the Shulchan Aruch (58:6) and most poskim, Chazal are only lenient concerning the Amidah and permit one to recite it until chatzot. The reason for this is that, in principle, a person is allowed to pray Shemoneh Esrei as a voluntary prayer (tefillat nedavah) at any time. Therefore, in any situation of uncertainty, one may pray. However, Birkot Keriat Shema may not be recited as a voluntary prayer. Hence, one who was late and did not recite them within the first four hours of the day cannot regain them. According to the Mishnah Berurah, if his reason for missing their recital was due to circumstances beyond his control, he is permitted to recite them until chatzot (Bei’ur Halachah there).
. There are those who wish to say that the Ashkenazic custom follows the Gra, and that Sephardim practice like the Magen Avraham. Indeed, the Chida writes that where he lived the people were accustomed to follow the Magen Avraham and, in contrast, the Igrot Moshe, Orach Chaim 1:24 writes that in Lithuania they practiced like the Gra also regarding Keriat Shema. However, the Yalkut Yosef 58:3; 89:6 writes that the Sephardic minhag is not necessarily like the Magen Avraham. Additionally, in Ashkenaz, many were stringent l’chatchilah concerning Keriat Shema and practiced like the Magen Avraham (Mishnah Berurah 58:4).
.The Shulchan Aruch rules like the Rosh and the Tur who maintain that the time of Birkot Keriat Shema only lasts for four hours. So write the Ben Ish Chai, Va’era 5 and Rav Eliyahu in his siddur. That is also the opinion of the Yalkut Yosef 58:6, as discussed at length. The Mishnah Berurah supports his words on the opinion of the Rambam and the Pri Chadash who maintain that Birkot Keriat Shema may be recited the whole day, and the opinion of the Mishkenot Yaakov 77 who says that the law concerning them is like the law regarding the Amidah, that b’dieved they can be recited until noon. The Maharil writes that this is the custom (Ashkenaz). The Bei’ur Halachah deduces that in a case in which a person did not recite them due to circumstances beyond his control, he may recite them until chatzot. Although many Ashkenazic poskim rule stringently like the Shulchan Aruch, among them the Graz and the Chayei Adam, nevertheless many Ashkenazim also rely on the Mishnah Berurah, as written in Halichot Shlomo 7:15 and Ishei Yisrael 18:7.Yalkut Yosef 58:6 writes that concerning the matter of Birkot Keriat Shema it is permissible to practice like the Gra. Although this is a case of doubt regarding berachot and presumably it would be proper to refrain from reciting them during a time that is controversial, in accordance with the rule that states that when there is uncertainty concerning a matter of berachot, we are lenient (safek berachot l’hakel), nevertheless, since some poskim maintain that the time of Birkot Keriat Shema lasts the whole day and others say until chatzot, therefore one may at least be lenient to recite them until four hours like the Gra rules.