Re: How long does UVA protection from sunscreen last?

that's definitely an interesting - and timely - question.

As it turns out, there isn't any standard method by which one would test 
sunscreens: every manufacturer has their own version of testing to put the 
rating on the package. The testing doesn't take into account sweat, water, 
washing, uneven or thin application, or removal by contact with hands, 
clothing, or other items, either; it is unlikely that any sunscreen will 
last the several hours of exposure time implied by the ratings, so it is 
wise to apply sunscreens liberally and re-apply often.

As of this year, according to the non-profit Environmental Working Group, just over half of 
sunscreens have some degree of UV-A protection: before this year, the 
number was significantly lower. Even those products which do protect from 
UV-A don't necessesarily have the same attenuation as they do against UV-B 
or even necessarily block UV-A as long as they block UV-B. The FDA does 
not require sunscreens to provide UV-A protection, nor do they mandate a 
rating for UV-A blocking.

Given the huge uncertainty in actual UV-A protection in any sunscreen, the 
best option is probably to make sure there is UV-A protection from your 
sunscreen, be careful about sun exposure even when wearing sunscreen (wear 
a shirt or blouse, wear a hat that can provice shade for your neck, face 
and ears), and re-apply your sunscreen frequently.

I hope this helps!