SPF X 10 min + ftr
A simple rule of thumb is spf
x 10 min is basic protection (spf15=150min) but there are many variables.
If one is sweating, the material washes off. Some stick better than others, but the term waterproof means you can dunk an arm in the water up to two times & it still works, after three it can be gone & still labled waterproof.