I fill my 700x25 tires before every ride. They usually drop from 95 to 85 psi.
My commuter's 700x35 tires get filled on Sunday and Wednesday evenings for my Monday, Tuesday and Thursday commutes. They lose ABOUT 5 psi from 90 to 85.
Beach cruiser tires are filled to 40psi whenver they get noticeably low, around 20 psi.