Each exam period is three hours. Exams go from 0755-1055, 1330-1630, and 1930-2230 (one of those slots on any given day).
Not all exams will take all three hours. The sooner he finishes, the sooner he'll be able to leave. I've seen plebes crush the Chemistry exam in two hours; the AP chemistry exam was far more difficult than the SC112 final.
Just scheduling a flight now? The exam schedule has been out for over a month. In my company and presumably others you are supposed to submit an itinerary for your leave travel up to your company officer several weeks before you actually go on leave......talk to your plebe, he/she better know by now what the last final exam is for him/her and should be able to gauge how long it will take to finish