Depends on the place where semen is left. On a cloth it dries fast but if collected in hand, you need to wash it off. Collecting semen in a glass bowl will take few hours when it becomes colorless
How Long Does It Take For Semen To Dry?
It also depends on how much there is, and the environment. If you busted a big load in a shirt and left it in a cool room, it could stay damp for several days