Just find out how many mill amp's the servo's and motor draw respectively, then its just a case of doing the math's - (someone correct me if I'm wrong), but my understanding is that if you have a system that draws say 1ma (hypothetically), a 3000 Mah (mill-amp-hours) battery will last 3000 hours, where a system drawing 5ma will last 600 hours (or 1/5 the total).
This video should help: http://www.youtube.com/watch?v=ZJHlJYDJohc