does istakhara vary from time to time.I mean if someone make lot of prayers and dua for something he wants while his istakhara is nagative.

Istakhara is to seek guidance from Allah. The result Istakhara is actually the guidance of Allah. His knowledge is, Absolute and Comprehensive. If the result of the Istakhara is negative, that is based on the prefect knowledge of Allah. The possibility of the condition changing cannot be ruled out as the power of Dua is equally great. However, we do not know the exact outcome of our dua and therefore advisable to exercise precaution and not commit on major issues without being sure.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai