Fajr begins at subh saadiq, true dawn or the beginning of twilight, when the morning light appears across the full width of the sky and ends at sunrise.
The time interval for offering the Zuhr starts after the sun passes its zenith and lasts until 20 min (approx) before the call for the Asr prayer is to be given.
The Asr prayer starts when the shadow of an object is the same length as the object itself (or, according to Hanafi school, twice its length) plus the shadow length at zuhr, and lasts till sunset. The recommended time to pray Asr is before the sun starts to turn orange.
The Maghrib prayer begins when the sun sets, and lasts until the red light has left the sky in the west.
The Isha prayer starts when the red light is gone from the western sky, and lasts until the rise of the "white light" (fajr sadiq) in the east.