Hi!
Sorry for my slow reply. Regarding lifespan, it does depend on the saddle and the rider (how's that for the most vague answer possible? lol!). I don't want to make it sound like I am bashing any particular model, but I have seen a couple of products that don't hold up quite as well as others. I've also seen some companies have some bad product runs but they supported the rider and provided a replacement.
Heavier riders, by nature, will tend to make a saddle "break down" a bit more quickly than someone who is lighter. The choice of materials in the saddle will also matter - not only the padding, but even the underlying structural pieces, and things like rail length - a short rail doesn't have as much adjustment, but it does put the support points closer together. I've seen some very inexpensive saddles that are basically just plastic that don't last long at all, and others (WTB Comp series comes to mind) that, while inexpensive, seem to hold up pretty well. On the other end of the spectrum, I've seen some relatively expensive saddles that broke down pretty quickly.
One rider I had worked with a few times a couple of years ago was on a $40 Forte (the Performance Bike house brand). He was also about 280 pounds. He rode almost daily, with a century just about every weekend. I think he got about 4 months out of it before it was noticeably bent/bowed down in the middle. But, it was comfortable for him, as it had taken to his shape. He came back with a new saddle - the exact same brand and model - and it didn't feel right when we installed it because it was new. Kind of like Brooks saddles - they are about impossible to just offer as a demo because they almost immediately start to become one with you. So, there are plusses and minuses.
There are even plusses and minuses within the plusses and minuses - for example, if you have a saddle that has worn to you, I can look at it and tell you which leg is longer. White saddles with graphics on the side of the nose are especially nice for this purpose.
Specifically speaking about the Mistica - I feel like it is a very stout saddle. I've seen a few that have had maybe 3000-5000 miles on them and they are holding up well. I will say that Fizik's products do feel like they are well made - they aren't cheap, but when you hold one in your hand, they feel like they are high quality. I don't know if that's a very good metric - it's not like you're buying a saddle to hold in your hand all day - you're looking for an answer about if that saddle is going to feel good day in and day out, under your pelvis, for years to come.
I don't know if I answered your question very well or not, but I hope that's some food for thought!
Travis Rassat
Vector Cycle Works Noblesville, IN
BikeFit Instructor | FMS | F.I.S.T. | IBFI
Toughman Triathlon Series Ambassador