The criterion in the times to make prayer is sunrise and sunset. According to this, fajr prayer comes in as soon as night is over and the whiteness coming form the east moves up and gradually spreads out through the sky. The ending time for fajr prayer is when the sun comes out. It is at this time that the window for making the Morning Prayer has ended and the prayer has become qaza.
The time for Dhur and Asr prayers:
If someone were to vertically put a wooden stick or metal pole in the ground in such a way that when the sun rises in the morning, the shadow of this pole falls towards the west, and the more the sun comes up the shadow lessens; then once this shadowed has reached its minimum and has slightly begun to move towards the east, this is when Dhur prayer first comes in. And after having made Dhur prayer (four rakats or two rakats (traveler's prayer)) in this time which is specific to Dhur prayer, then the time for Asr prayer begins. The end of Dhur pray is when there is only enough time before sunset for making either four or two rakats [of Asr prayer], and after that Dhur prayer is then considered qaza. The end of the timeframe for Asr prayer is the beginning of Maghrib prayer; meaning that the amount time it takes to make Asr prayer before Maghrib is specific to Asr prayer and after that it becomes qaza.
The time for Maghrib and Isha prayers:
Maghrib prayer comes in after sunset; after the redness in the sky which comes from the east, is no longer visible. This is the starting time for Maghrib prayer. Maghrib prayer ends when there is only enough time left before midnight to make four or two rakats of Isha prayer. The time for making Isha prayer starts after the necessary time has passed in order to complete the three rakats of Maghrib prayer. Isha prayer ends once it has become midnight.
It is necessary to mention that when it comes to fajr and Dhur prayer there is no discrepancy between the Sunni and Shia. As for Maghrib prayer, it can be made approximately ten to fifteen minutes after the time that Sunnis consider the beginning of Maghrib prayer.
You can also refer to various Shia centers in