What product? What pad? What are you trying to do? Need more info. to answer your question. Get VERY specific and we can help you. The knowledge in this place is outrageous.
sk24max- The generalized, short answer is "as often as you need to" and I'm not being facetious
With polishes you don't want the pad to go dry, lest you cause new marring. And since some/most polishes break down (lose their abrasiveness) you sometimes need new (non-broken down) product on the the pad even when it's not dry.
With LSPs you also want to avoid running the pad dry, but you generally don't need as much product as with polishes.
But yeah, the more specific you are about the situation the more specific the answers can be.