A little bird told me the answer is "Who?" but there are currently several independently conflicting studies on the matter. Technically speaking, one mustn't necessarily lick a pop at all, simply leave it in one's mouth for a long enough period of time. This naturally affects the number of licks required, removing potential future lickings the longer a pop is held inside a mouth, or possibly twirled against the inside of one's cheeks. Assuming a pop is held outside the mouth to control these factors, it still depends on ingredients, weather conditions, saliva production, the coldblooded or warmblooded nature of the consumer, tongue stroke technique, number of participants, and the inherent texture of said tongue(s). It's a surprisingly neglected field at first glance, but understandable once one begins analyzing the various complications. The Equestrian Food and Drug Administration mandates a bare minimum of three licks be required before safest biting by mature Strigimorphae, but their regulations are only loosely enforced, especially when accounting for varying beak strengths.