Once at the end of the day. Also run CCleaner at the end of the day to remove other unnecessory files. DON.
Hmm, browser history every 1-2 weeks I think. Although there are times when I forget to erase it for months xD
I have set up browser in such a way that immediately my browser history will disappear. I think this is better for your system health...
On my work PC usually every week or two, but I frequently forget and can go months without doing it. Not like it matters, my boss downloads more porn at the office than I do lol. At home, usually only if I'm worried about something or the laptop starts slowing down. Usually once a year I do a complete clean out of it though (deleting old programs, images, etc, and defragging the hard drive).
I sometimes need links of sites/threads of DP which i had visited a week before, so i usually do not clean the history.
Ordinarily I clean all temp files and history every day or every other day with both CCleaner and Webroot Window Washer... I would highly recommend CCleaner to all Windows users as a excellent free must have cleaning application...