If I had to hazard a guess, I'd say about 8 or more, but I don't think I've ever left it more than 2 hours without other use.

I do think it'll fit what you want pretty nicely (Maemo Mapper is awesome, BTW), but you might need extra battery capacity; there are some AA/AAA or rechargable Li-* battery packs available that you can connect to the charging port, while its in your pocket. If I went roving like that, I think I'd get one.