Something else...if an engine develops a small oil leak on the same side of the engine under the timing belt cover, it will end up on the belt and that will be a factor in belt life too.

I equate it to driving with old tires. You can sometimes go way past the mileage but age catches up to them even though the tread may look good. It's just not worth the risk. Either save the cash to get it done, or if you're a pretty capable DIYer the belt doesn't cost that much. To really do it right, replace the water pump and all the belts at one time. It's better to take care of the basics before you mount that airplane wing on the back