Anshei Knesset HaGedolah instituted praying three prayers every day, and established their times to correspond to the Tamid offerings. The morning Tamid was brought starting from amud hashachar, and based on that, the time of praying Shacharit l’chatchilah should begin at amud hashachar. Nonetheless, the Chachamim say that it is proper to recite the Amidah after netz hachamah, as it is written (Psalms 72:5), “Yera’ucha im shamesh” (“Let them fear You from when the sun [shines]”) (Berachot 9b). B’dieved, if one recites the Amidah from amud hashachar, he fulfills his obligation because he prayed at the time when the morning Tamid offering was brought (Shulchan Aruch 89:1).
The most praiseworthy time to pray follows the manner of the early chasidim (devoted ones) who prayed at vatikin, for they would recite the Amidah at netz hachamah.
The time to pray the Amidah lasts four hours according to Rabbi Yehudah, for that is the last time that the morning Tamid offering may be brought. Even though, in the opinion of the Chachamim, the time to offer the Tamid lasts until chatzot, the halachah follows Rabbi Yehudah, since in Masechet Eduyot, in which all the mishnayot were chosen as halachah, the opinion of Rabbi Yehudah is brought. Therefore, the time to recite Shacharit lasts until the end of the first four hours of the day (Berachot 27a). Even so, the Chachamim’s words were not rejected completely, and if four hours passed and one did not yet recite Shacharit, he may b’dieved pray until chatzot. Although his prayer does not have the value of prayer recited on time, nevertheless, it still has certain value (as clarified further in halachah 11).