The lifespan will depend entirely on the number of Writes the drive will incur. The first thing you want to do once the Os is installed is to reduce this. If you have enough ram and are comfortable with nos wap go for it. If you want the backup i would suggest pushing swappiness all the way over so that it is used only if there is no ram.
Most SSD's will give you a 3 year warranty. Samsung with their Evo 850 drives and the new vnand are offering 5 year warranty. (they are also wicked fast).
That being said. with minimal writes I have seen older SSD's last for much longer than their supplied 3 year warranty (I have one that is pushing 6 right now).
With an SSD the general user experience will be pretty good, but Anytime you run updates it will still crawl, regardless fo the SSD :-)