I get 36 hours normally with the stock Sense 2.1 ROM and Unity V10 kernel. Overclocked to 1Ghz with the standard governor.
I use it quite lightly though, 10 to 20 SMS, bit of internet / Facebook and 30 mins FM radio in the morning. Also accounts sync automatically, mail picked up every hour, data always on. That was with the stock battery, with the Anker 1600 mah I get to 36 hours with about 30% to spare!
On a side note, underclocking below 245Mhz has been know to cause problems. Standby of Death (or Sleep of Death), and possibly dropped SMS etc.
XDA2 -> XDA2i -> SE K750 -> SE K800 -> O2 XDA Orbit 2 (HTC Polaris) -> HTC Touch Diamond 2 -> HTC Touch Pro 2 -> HTC Desire Z -> HTC One
Remember to press thanks if you find my posts helpful.