It is recommended that the belt be changed at 90K miles. Does anyone know what the average miles is that he belt actually fails?

Specifically no. Generally, any miles beyond the recommended 90K miles is a roll of the dice. That said, I regularly roll the dice. I had an 87 4 banger a while back that was scheduled at 80K miles. The belt slipped taking out two teeth at 116K miles. That was my first experience with a timing belt. I've changed several TBs at or near the recommended mileage and without fail am amazed how pristine they look. Considering those two points - what am I saying? Would I rush to a shop when it hit's 90K miles? No. Could I wait till I hit 100K. Sure. Would I let it hit 110k miles. No way in hell.

FYI - You have a non interference engine meaning if the TB bites it, your valves, cylinders, etc should not be damaged. You will, however, not be moving from the spot it happens whether on a quiet country road or an interstate highway, in quiet suburbia or somewhere where you're constantly checking your six.