How long does it take basement floor paint to thoroughly dry? Should I then seal it with polyurethene?

It's been on 48 hours now. It's a very dark blue and seems to show dust very badly. If i walk on it, you can see footprints. I don't know if it's just not completely dry or if I need some protective layer over it. Please help. It's a gorgeous color and I don't want to change it to grey if I don't have to. Thanks.