It depends on a number of factors. There is no real way to measure it:
1. Age of the battery. Over time the battery will hold less charge and run out of charge quicker. Newer batteries last a while longer than ones that have been used for a year.
2. Not all games use the CPU at 100%. Some games (or the XMB for instance) use very little CPU time, and some games constantly hammer the CPU which will only drain the battery faster.
Everyone's batteries last differing lengths of time depending on what they are doing. Personally I've never run out of charge, and I have no idea how long the battery lasts in mine as I don't really play on it for 3 hours continuously, only when I get spare time.
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