Quick internet search to rogaines page :
"In the evening, follow the steps above to be sure you apply Men’s ROGAINE[SUP]®[/SUP] Foam correctly. Make sure that you apply it early enough before going to bed to allow it to dry completely."
I usually give it at least 10 - 15 minutes, since it dries so fast, and because i always forget until right before bed. I'm new at it though.