That's tough... It probably varies a good bit. I would think 80-90k is a good interval. It could need it as early as 60k and as late as 120k. Look on the inside of the belt (where it goes around an idler) for tiny little fractures in the rubber.
Some of the things that make a difference are how you drive and where you drive: Hot climate, short driving loads the alternator and the engine cooling fan; especially with the A/C running full bore. Temperate climates with little A/C, during the day etc is easier on the belt. Extreme cold forces the belt to round all the corners when the belt is in a hard, brittle condition.