Unforunatly, I am a Professional Painter. Been doing it for over 16 years and have used alot of exterior(house) paint. It takes up to 30 days for any latex paint to fully cure. Eventhough it feels hard in a couple of days it really isn't ready to take a real beating for 30 days. Just a little trivia for the guys wondering about it durability.

Just as Fryfly mentioned that it seems to get tougher as it goes along,that's just the paint curing.