I have read from several sources online* that any suncreen, even physical sunscreen, must be reapplied at least every 2 hours (or more frequently if you've been swimming or sweating).
For chemical sunscreen, I understand this—chemicals like avobenzone protect you by breaking down under UV radiation. The longer you stay in the sun, the less protection these chemicals are able to provide. However, physical sunscreen forms a physical barrier—why is it necessary to reapply it, assuming you haven't been sweating or swimming? Shouldn't it still be on your skin?
I should note that neither of the two packages of physical sunscreen I have make any mention of "mandatory" reapplication in the absence of excessive sweating or product wear. That being said, I've also seen a bottle of chemical sunscreen (made for the face) that makes no mention of it, either.
I should save you all some time and note that Wikipedia indicates that the best action might be to reapply once within two hours of your first application, and then only reapply as product wear occurs:
Contrary to the common advice that sunscreen should be reapplied every 2–3 hours, some research has shown that the best protection is achieved by application 15–30 minutes before exposure, followed by one reapplication 15–30 minutes after the sun exposure begins. Further reapplication is only necessary after activities such as swimming, sweating, or rubbing/wiping.5
More recent research at the University of California, Riverside, indicates that sunscreen must be reapplied within 2 hours in order to remain effective. Not reapplying could even cause more cell damage than not using sunscreen at all, due to the release of extra free radicals from those sunscreen chemicals that were absorbed into the skin.
This still doesn't address the issue of physical sunscreens. Am I avoiding the need to reapply by wearing physical sunscreen? I don't mind the paleness of physical sunscreen as long as it's "buying" me the right not to have to worry about reapplication.
*Most of these sources have blog or forum comments, because I don't see many normal articles talking about this subject. However, from a quick Google search, here are a few other websites that seem to lump all sunscreens together into the "reapply-every-two-hours" category:
Melanoma Foundation (thanks, Oddthinking♦), Fab Over Forty, Dear Doctor.