Depends how busy I am. Sometimes a few minutes, sometimes a few hours. Sometimes if I'm building an exciting campaign I'll "pause" the play and resume it later. This usually means that the toys are left on the playroom floor overnight.

But unless I know I have more free time to spare, I usually play a "stand alone" story with my toys with a simple plot that can be finished quickly. If I have more time I'll try longer and more epic "complex" stories.