You can go well past 70-80K...they're good to over 100K. The earliest I've seen one break (guide pulley killed it, didn't break) was 108K...seen them go almost 120K.
When you do it, replace ALL the guide pullies and tensioner. They will not last to the next belt -- when they die, they eat the belt -- and it's the same amount of work to do a pulley (with a chewed belt) as it is to do a belt.
Well I can attest that timing belts can last until 123984. And yes that's the current mileage on my car. I checked the belt not too long ago and it was fine. I couldn't tell you what a bad pulley would look like.