PM me for some sources on what's okay; I don't have enough posts to post links yet, but I found the cutoffs for swimming pretty easily. It varies a bit by body chemistry, but the equivalent of 3-6 cups of coffee per day is enough to push some people over the limit.
EDIT just kidding about PM I guess; need 10 posts to be able to do that. Google "swimming+"FINA lists banned substances" and "Caffeine renal clearance and urine caffeine concentrations during steady state dosing. Implications for monitoring caffeine intake during sports events." First is on swimmingworldmagazine, second is on ncbi.