Depends on the dive. If it is a drift dive, there is no turning back, so I try to make sure I begin my ascent at around 800-750 or more depending on depth
for a dive that would require going back to a boat or shore, i usually turn back at around 1500 PSI and try to make sure that i'll be able to surface with 500 PSI left in the tank, so typically start ascending between 800 and 750 PSI depending on the depth and estimated safety stop time.