Two weeks ago had an interesting conversation with our dermatologist about sunscreen. First was, you don't need anything more than a "30" rating. Higher ratings are an excuse to charge more.
He said ALL sunscreens should be reapplied every 75-80 minutes, but very few companies will state that on their products because Americans assume that means the product is not as good as the "others." (Note: I did buy a product last week that states how long it lasts).
In Mexico, all products state the re-apply rate, so natives of Mexico are used to it. At the beach in Mexico, he overheard one American family reading the label on the locally purchased sunscreen say, "This stuff is not very good. It says you have to reapply it every 75 minutes!"
He believes that it will take legislation to mandate that U.S. manufactured sunscreen products have labels that state you have to re-apply every 75-80 minutes. Even using "waterproof" sunscreen doesn't extend that more than a few minutes.
He says 1 ounce will cover your body, so if you're at the beach all day you need a 6 - 8 ounce bottle of sunscreen per day per person.
And anything you're getting frozen off now came from sun exposure 20 years ago.
So I guess at some point in life, you can forgo the stuff... !
So, I guess I'm on the once-a-year schedule. Too bad. 
