Olds original concept was to use the fan to circulate the heat distributing it more evenly throughout the cabinet, and to eliminate the need to rotate the racks during a cook. The idea of placing it in the back and above the drip tray, was thought to be the ideal spot for circulation. A few members have placed them below the drip tray on the right wall blowing air towards the generator, but I can't recall how well these performed, or if any performance posts were made on this configuration.
I've purchased the parts, but never got around to installing a fan. Mainly because my back wall warped. If you read the reviews from people who have installed the fan a lot of people say it works, and there are others who say there was no improvement.
If you install it as Olds originally posted, it will run as long as it is plugged in. There is no reason why you couldn't plug it into a timer, or add an on/off switch, or just unplug it when it is not needed.