Hasn't been updated in 9 months and may/likely not work in WoW 2.4, but FuBar_CombatTimeFu works well currently. Makes a timer that continually shows in FuBar (minutes/second/fractions of a second) which resets & starts when you enter combat and stops when you exit. Shows you the total in-combat time after each fight. That's it.
Not the most efficient mod--it uses CPU time--but it works. For something more and if you are into programming, what Rabbit suggests is the ticket.