Good question. I've never waited - I usually apply two light coats, pretty much back-to-back. The reason for the second coat is just to make sure I didn't miss any spots. I'll apply to both wheels on a side, then remove, then repeat. Depending on the weather, I seem to get 4-6 weeks of durability.
Some folks have mentioned that PB wheel sealant - and Wheel Wax for that matter - make their wheels look better. I haven't found that to be true. In my experience it doesn't do anything for the appearance of the finish, but it sure protects it.