- «Час» is used with numerals ending in 1 (but not 11).
- «Часа́» is used with the numerals ending in 2, 3, 4 (but not in 12, 13, 14).
- «Часо́в» is used with all the other numerals.
@Shivaadh - "Two hours" will still be два часа. The numbers 2, 3 and 4 use genitive singular, and 5 and above uses genitive plural. The numbers themselves and the words they modify can also adapt to other cases (for instance, "к двум часам мы завершим работу" (we will have completed the work by two o'clock).
It's "часА" after "два", "три" and "четыре", and "чАса" in all other instances of the genitive. Don't ask why; I never even realized we are doing it until you asked
(well, actually, I believe, it's for historical reasons. There used to be a partitive case, but it's mostly merged with the genitive. This is a remnant of it).