It depends on so many variables, student aptitude, instructor aptitude, type used, frequency of training, weather, student's attitude (end-goal orientated or wants to enjoy the training for its own sake).
Comparison with other students is inevitable, particularly within a busy school, but in my experience quite detrimental and totally irrelevant. But to answer your question, I've seen 39 hours up to 100+ hours.
(And I've also known of one or two tenacious students like CF mentions!)