The current uptime on my Treo 600 (running Palm OS) is 35 days. From a single charge. It says 35% left. Although it was on standby for >99% of the time, so I'm not sure it counts. The reason it is 35 days is because I forgot about it and the battery ran out. When I used it regularly I had it running without a reboot for 2 years since I replaced the battery. The battery was 5 years old. That's how long it was running without a reboot before the replacement.

People were making stable operating systems before Maemo

I reboot mine at least once a day. Sometimes more than that. It depends on how much I manage to resist fiddling. Installing a new application usually requires a reboot to force the redraw of the new icon. Which is a bummer, I really believe it shouldn't.