From Wikipedia

"Three independent research studies have been carried out to determine the answer. Engineering students at Purdue University devised a "licking machine" that showed an average of 364 licks are needed to get to the center. Using 20 human volunteers, the figure was 252. A doctoral student at the University of Michigan created another licking machine and came up with an average of 411 licks."

The page on wiki about tootsie pop
http://en.wikipedia.org/wiki/Tootsie_Pop