Don't have the manual on hand, but seems the recommended interval for the serp belt is 60K. That said, plenty of people go every 70K with the timing belt, and plenty of people change it more frequently. I've heard of folks doing it at 30K in very hot, dry climates. Ultimately, it's a matter of personal preference/comfort level. If it snaps, it's not as catastrophic as a timing belt - but it's still not good.

I do it at 50K because I live in a dry climate and because it's easy for me to remember. Way I see it, the belt costs ~$20, and it takes ~10 minutes to change. You could even do it while you're waiting for oil to drain when you're changing your oil, and you'll still have time to go enjoy a beverage of choice. (If you take that route, careful about getting oil on the belt.)

If you're up in the 140K mile range, you might want to consider putting in a new tensioner/tensioner pulley and idler pulley, too. Do a search of the archives for all of these procedures...there's been plenty written about how easy it is to change the serp belt.