How Often Do I Need to Reapply a Physical Sunscreen for the Face?

This is a question that I've extensively searched the answer to and have not found a definitive answer. I've reached the following conclusion. Chemical sunscreens are like sponges. They can only absorb so much UV rays and must be reapplied after a certain amount of time or sweating/swimming. But physical sunscreens are like shields: the rays bounce off your face so it doesn't need to be reapplied unless it's been eroded by sweating or swimming. However, it doesn't hurt to reapply, especially considering that physical sunscreens don't cause free radical damage like chemical sunscreen. The following link might be of further help. http://www.skinacea.com/sunscreen/physical-vs-chemical-sunscreen.html#.UOzxdW_hqgQ