I'd do a rough estimation on the conservative side like the poster said above. Use a general formula like:
(average # of events per month) x (average # of hours per event) x (# of months you were involved)
if you have more details, then you can weight the avg # of hours per event based on frequency of different types of events that have different amount of hours. You don't need to be very very specific, just say something like active fraternity member, involved in volunteering for 3 years for a total of around X hours. Typical activities include X, Y, and Z about _ times a month during school year.