I don't have mine on a timer, because of the tendency around here for evaporation to take the water level too low. During swim season, I turn it on in the morning when I get up, and off when I get ready to go to bed, so it runs somewhere around 12 hours. During the winter I run it overnight when the air temps are below freezing, but otherwise just 2-3 hours a day to circulate chlorine when I need to add it (my pool never closes).

When clearing up an algae bloom or other water problem, the filter needs to be run pretty much 24/7, but otherwise it only really needs to run enough to turn over your pool's water volume once a day--for most pumps/filters, 6 or so hours a day is plenty.