I don't know how long they'd last, but I can only think of two types of failures that I've had. The most common is loss of fuel, I dont' know the reason why, either a leak or it accidentally gets pressed in my bag. It doesn't happen a lot, but it's happened a few times.

The other type of failure is leaving it lit too long. The spark wheel heats up and melts the plastic, and the pressure from the flint springs push it out.

They're cheap enough that I would just double them up wherever I need them. The chances of two of them going bad are pretty slim.