please let me know the timings of fajar salat i.e. how many minutes before sunrise we can read it, suppose the sun rises at 6:30am

In the last part of the night, at the approach of dawn, some whiteness can be noticed on the length of the horizon towards the east, i.e. from the direction in which the sun rises, after a little while witeness can be noticed on the breadth of the horizon. This whiteness begins to spread very rapidly, after a while it becomes completely bright. From the time that this broad whiteness becomes visible, the time of Fajr commences and remains until the rising of the sun. the moment a small portion of the sun appears, the time of Fajr ends. (Tahtawi pg.175)

However, it is preferable to perform the Fajr Salaat well before sunrise, i.e. after completing the Salaat, there is still time to perform Salaat again if due to some reason, the Salaat was invalid. It is not permissible to perform any Salaat at sunrise. Therefore, whilst performing Fajr Salaat, if the sun rises, one should terminate his Salaat and make Qadhaa later. (Fataawa Mahmoodiyah vol.10 pg.261)

Once the sun sets, Maghrib times commence. Maghrib time remains as long as the redness on the western horizon remains. However, Maghrib Salaat should not be delayed to the extent that many stars begin to appear in the sky. To delay it till such a time, is Makrooh.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai