I've had some of mine going for 6 or 7 years with no problem. On the other hand I've gotten some used ones that didn't register correctly with a watt-meter. It never hurts to have a back-up. If you keep an eye on ebay you'll find them for a good price.
Unless you live in the middle of nowhere and need a few days to get one, I wouldn't worry about it. 1 could last you a lifetime or it could also shit out on you in a day.
It's always good to use a 24/7 thermometer over every heat pad, and of course always plug them into a thermostat. In our terrarium setups, we use reptile carpet as our substrate, place the UTH in one of the rear corners, tape the probes for the stat and LED thermometer with a small piece of masking tape to the carpet over the UTH and under the warm cave....
That way we are always monitoring the temp of the UTH/warm hide to be sure all's working correctly. I like the Exo Terra LED thermometers. Very accurate and about 10 bucks.
I've got some at home that are 10 years old and going strong, and within the stores they regularly last for years and years, and that's with rough use - we regularly slide cages on top of them and over them, which can really shorten their life span. Even with rough use, they still last easily 5 to 10 years (pretty sure in our 17 year old Oceanside location, some of the original heat pads are still being used!!).
Probably just the individual pad its self. Im sure some brands have a average lifespan but i mean, as its been said, Some may last forever, and others only a day.