Your Mileage May Vary. But on my machine (TM), the battery life substantially dropped on my 2010 MBP after installation of ML. By substantially I mean really really badly. From 4 hours previously to down to about 1.5 hours.
Surfing around about that problem, I did find the programm gfxCardStatus which allows me to define which graphics card I’m using. Interestingly, I was using the NVIDIA GeForce GT330M instead of the Intel HD Graphics. Using gfxCardStatus, I can switch that to use only Intel HD Graphics – which is totally sufficient for my day-kind-of-work. And yes, the projected battery life went back to where it was before.
I have the impression that with ML (it wasn’t the case with Lion, at least not for me), something makes my MBP use the external graphics card all the time, with no reason to do so.
gfxCardStatus, moreover, allows me to choose to allow having the external graphics whenever I’m on power supply, and to use the internal graphics when on battery.
Actually, I’m going to run on the internal graphics most of the time anyway – when watching flash videos, for example, finally the MBP heats up a lot less, and also remains silent.
I’m no gamer, so I’ve no need for that extra boost of graphics power anyway.
But, as I said, YMMV.