The time for the morning Shema begins when there is enough light to recognize an acquaintance from a four-cubit distance. The time for Shema ends after the third hour of the day, though l’chatchila it is forbidden to delay it. It’s best to read the Shema K’vassikin. If you do, you’ll receive a lot of reward. (ShA 1 – MB 3)
The berachos of krias shema also should not be said before this time. (MB1)
If you didn’t manage to say krias shema before sunrise, try to say it as early as possible. (ShA 2)
Whoever recites the Shema without tallis and tefillin is as if bearing false testimony upon himself. It is justified to delay krias shema so you can wear them – just don’t allow the zman to pass. If you are doubtful that you will have a tallis and tefillin before the sof zman, do not delay. Say the Shema without them, and later when you put them on, say the Shema again or any other parsha, even a chapter of Tehillim. (MB 5)
The Shema must be finished before the deadline. (MB 5)
On Shabbos and Yom Tov, it is common that the minyan starts after the deadline, so act wisely and gather a minyan early so you can read the Shema with the minyan before the time limit. If you can’t, at least recite the Shema before going to daaven with the tzibbur. (MB 5)
If you are careful to read krias shema k’vassikin, you are allowed to daaven then by yourself, if there is no Vassikin minyan. (BeH “umitzva”)
The very earliest you can read the morning Shema is from dawn. However, you shouldn’t do this unless you are in a situation beyond your control that you won’t be able to concentrate to read the Shema (even until “al l’vovecho”) later within the time limit. (ShA 3)
The berachos of krias shema are like tefilla and therefore their time limit is also at the end of the fourth hour. After that, saying the berachos will be considered l’vatalla. The custom is to allow one to say the berachos until chatzos only if the situation earlier was beyond your control. (ShA6 – MB 26, BeH “kor’ah”)
Even if you miss the time for krias shema, it is still good to read it in order to accept upon yourself ole malchus Shomayim. Besides, the third paragraph is a mitzva d’oraisa of remembering yetzias Mitzrayim that applies all day long. (ShA 6 – MB 27)