A failing tensioner or idler pulley will chirp a little on cold starts to warn you. Probably could go for weeks on it. A continuous LOUD squeak is either a pulley that will fail soon, or a worn/stretched belt that's slipping and needs replacement or has become contaminated with a fluid and is slipping. You could also have an accessory that's binding (water pump, AC, power steering pump) and causing belt to slip.
The risk is if a pulley locks up and you throw/shred the belt, it can do damage slapping around in there (destroy cooling fan, coolant expansion tank, etc.) and when you lose the belt, the water pump stops and the engine overheats. You don't want that.
Last edited by ahull; 11-30-2012 at 05:13 PM.