serpatine belt
 

Do you replace at a certain number of miles.

I replace when it starts to show signs of cracking

I would never let it go more than 50,000 miles.

I keep a spare belt in the jack compartment, they`re cheap enough

Whenever you notice your engine temp going up, and the steering gets really heavy, it's time to replace it...

Hopefully you have a spare with you, it only takes five minutes to change one.

Or, if you don't like surprises, when it starts looking all cracked.