I also have 2 batteries, since nokia replaced my defective n800 with a brand new unopened box (thus I also have plenty of styli and an extra cloth case). After two years, they're both starting to show their age, though a single battery can still easily make it through a day of heavy mp3 and radio listening and moderate web browsing. Since March 2007, I've yet to have a day when both batteries ran out.

Edit: I'd like to add that when they were new, each of my batteries could watch 2-3 hours of mediastreamer content at medium brightness. Now they are about 1:20-1:40 at low.