It's in the FAQ thread and in a couple other areas. It actually isn't a very frequently asked question.... so it wasn't included.
That varies depending on several situations, but on average, you can expect about 4-5 hours of runtime for a warrior, 3.5 - 4.5 for a champion, and 3-4 for a hero.
Edit: To clarify, runtime means the saber powered ON. If you have it off, or only on periodically, it'll last days, even weeks.
Recently though, those times seem a little on the short side. Some people have been getting 5-7 hours runtime out of their sabers.
Batteries in these are like batteries in everything else; it just depends on how well you maintain them, for how long they're going to last. If you sit and let one be dead for an extended period of time, eventually the cell in the battery is going to die, and be unable to hold a charge; that's just the nature of rechargeable cell batteries. If you maintain the batteries well, they'll last a really, really long time.