Trip cycle or trip endurance are the measures that you are looking for. The first is the number of times you can trip and assume that damage to the part has not occurred, the second is the length of time the trip state can be active for before the part is damaged.
Many factors will also determine whether the PTC becomes damaged or not, heat, humidity, age, etc. and it is impossible to determine when the fuse will fail. Normally the point of putting one of these in a system is that the replacement cost of the PTC is less than the replacement cost of the components it is protecting. Think of every trip as one get-out-of-jail-free over traditional fuses.
These are a last line of defence and don't assume that the fuse will always trip to protect your circuit. I hate to think about the number of times a $100 tweeter in my stereo has blown to save the $0.10 protection fuse.