To see what your battery's stats are.
Finder--->Apple Icon-->About This Mac->More Info->Contents>Hardware>Power>Battery Info.
Things to note: Full Charge, Current, and Cycle Count. I have two batteries, one with a high cycle (nearly 300) count that gets me about 2-2.5 hours of up time. I have another battery with a low cycle (nearly 50) count, that gets me about 3-4+ hours of up time. All of this is dependent on what processes I am running FCP and LiveType or compression programs- high usage. M$ Word and iTunes- Low Usage. Running Backlight along with an OpenGL screen saver- High power usage. I can't see what a theem would do to your CPU or HD usage.
The upshot, don't buy a used battery with out having the seller give you these stats. If a battery has a high cycle count and a low full charge capacity, then don't buy it. I would likely suggest getting a new battery to use when the first one goes. But that's me.
Edit: I was just PMd on how to find the battery stats. Do noted above and if you still can't find the stats then do the following:
System Profiler->View>Full Profile (option 3)
Then
System Profiler->View>Refresh (option R)