Inspect regularly, change if signs of wear are detected, changing occurs roughly every 30k miles give or take because usage and environment varies. If there are signs of wear, it should be changed regardless of when it was last changed, and if it was last changed recently, you know there's a problem with premature wear.
In your case, if the belts look good, your trip would have to be maybe 20k miles before being concerned that a good looking belt might be a source of failure. I worry more about tires for long trips before I concern myself with drive belts that look fine.