It depends on a few things... how high my sales have been with that sponsor, how busy my traffic is at the moment, and honestly, how easy checking my stats are.

Not to cheer for the home team too much, but Bedfellow's stats are extremely easy to check, so I check that all the time, and often make changes to my sites based on how much traffic I'm sending over, and some stats are a pain in the ass to check (having to go through four pages, enter two or three passwords/used names) and those irritating java-based reporting programs that don't spell things out very plainly. Those guys get checked less often.