Amazingly, this is the hardest question I've been asked about frogs!
It turns out that very little is known at all about the natural lifespan of frogs. Partially, this is because it's pretty hard to track a frog all its life! (I guess they havent figured out a good way to put little tiny collars around their necks!)
However,some records show that in captivity, many species of frogs and toads can live for surprisingly long times. They seem generally average somewhere between 4 and 15 years!
Recently I ran across a page where people were posting data about how long their species of
frogs had lived in captivity.
Other species which live to ripe old ages include:
REPTILES AND AMPHIBIANS IN CAPTIVITY - LONGEVITY HOMEPAGE: FROGS & TOADS