Until it's worn out. Since the rear tire wears out much faster than the front tire, it is not advisable to put a rear tire on the front. Your freshest tire should always be on the front because if a tire is going to go down because of wear, you want it to be the rear tire. This is what Sheldon Brown and many others around these parts have said, so I've stuck to it.

On the subject of car tires, I do a 5 tire rotation every 3,000 miles. It might be overkill, but A) I don't drive my SUV very often (3,000 miles a year tops) and B) my tires with approx. 36K miles on them are worn evenly and still have plenty of life left. (I used to drive a lot more...86 mile commute one way.)