I'd like to say "never". but, that's not really true. The option on the poll that had the highest option is "more than 100 hours". So, I chose that.
I do a reboot about once a month.
Whenever I get a hankering for a thorough cleaning and break out the q-tips and cleaner. I do some cache cleaning, clean up browser history, delete old sms and email, delete dumb pictutes from gallery, uninstall apps that were downloaded hastily and now are unwanted.
After cleaning the device, the SD card and the cache it just makes sense to do a reset (NOT a battery pull) and get all cleaned up and refreshed.
by the way... best method for reboot (on Droid)... slide open keypad and press and hold control-alt-delete. NOTE: "control" is the arrow key to the left of the "z".
PS: compare that to my experience with Blackberry: 3-4 times a day.