21 days. I used to reboot regularly, but there's no need now that I know the following:
1) - With simple launcher installed, new applications caused the home screen background to go white. Instead of rebooting, just re-load the background wall paper.
2) - Occasionally the built-in email refused to start, and email notification didn't work. Instead of rebooting, just start an xterm and enter 'killall enprocess'

I don't expect to reboot until the next firmware upgrade..