Disclaimer: I use KP47. Previously KP46, and previously stock kernel. My battery charging time has never been more than 2 hours (OK, according to battery-eye: Longest charge time 2:13). If your was needing 3 hours or more, you were either running some demanding program while charging, or you are using a non-standard charger and/or non-standard battery. You could install battery-eye and visually check what's going on with your N900. Powertop may also help, but it's harder to get any (useful) information from that.