I have an N810 which I tether to a Nokia E51 (3G connection)
With the following setup:
1. boot to internal memory
2. no SD in external card slot
3. no swap memory
4. only use QTNX to connect to a desktop computer, nothing else
I turn my N810 into a thin client, and do all the heavy lifting on my home computer. NX blasts any other remote desktop out there -- it's like I'm working locally!

Bottom line: I managed to get ~6 hours of almost constant use.