About 2 inches in the bottom while the rest of the front service is happening. Pour out the liquid and install them, leaving whatever hangs on. An informed user suggested this trick to me last year, and it is very effective. I use the gear lube since I live in an always-warm climate. The drive belt has been replaced at 30k, 70k, and 100k.
Belts always give up at the worst times, so putting one on while doing something else is cheap.
I keep an old one in the toolkit just in case one fails on the road. The spare gets pumped up to 40 psi at least once a year. A little tire foam on the spare and the rest of the lower stuff looks real pretty for a while. The engine compartment gets a bath a couple times a year. I take a pass through with a little 409 in a spray bottle, then hose everything out.