Its a fact that battery life is somewhat user variable.. depends upon your usage and how you have managed your settings. I suggest you read the reviews of the ROMs in the thread of the ROM which you want to try out. Reducing screen brightness.. regularly toggling mobile data and wifi , underclocking your kernel are some ways of increasing battery life.. On an average I used to get about 4 - 4.5 hours of screen on time .
Hope that helps you.
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