How long will an Apple SSD last?
A simplified answer to SSD lifetime would be: 
SSD is a Semiconductor devices build to last minimum 10 Years. Some of us still have computers from year 2000 or even before, thus older than 14 years. 

With that said, looking closer at the SSD, it is a flash memory type semiconductor device and could develop leaking currents (means the cells loose they charge (memory). 

There is difference in life (quality) of SSD's depending on manufacturer (Intel, Samsung ect.). To my knowledge, Intel is still the front runner on quality = life time, but Samsung is chasing them. It is really difficult to say who is the best, but one can use the warranty as a guide. For example Samsung gives 5 Years on the 840 model. That means it will last minimum 7-10 years, otherwise they would be out of business replacing all dead SSD's. 

Just to clarify, I am talking about the most common type of SSD (using the Single layer memory cells), the dual layer lasts even longer but it is much more expensive.

Apple, Crucial, OWC ect. do NOT make the actual memory chips. The buy them from Semiconductor manufacturer like Samsung or Intel ect. and make the SSD boards with they name on it.

However, do not panic, the drives have a self repair capability! 

They do that by simply replacing the dead cells. 

The problem is there is limited supply (reserve) of new cells.

One of the biggest life limiting factor is the amount of data stored. Let's say when the SSD is 90% full, it will become a problem soon, since the drive tries to rotate the cells to preserve/avoid memory leakage. 

Second life factor is the daily usage (read/write). Again, do not panic, a "normal" user will never see that problem.

All in all the SSD should last you longer than your computer screen or keyboard!

SSD are sensitive to heat (as any Semiconductor device) and will loose life if operated at elevate environment temperatures, means they love cold environment. Does not mean you have to move to Alaska :). 

Just to give you an idea. Why do you think Intel invented the so called "I" processors they call it Intelligent CPU. Great Marketing by the way. The actual reason is the CPU becomes too hot if it operates at 100% all the time. 
Same applies to the SSD chips, so keep them cool and do not overload your drive and it will last and last.

And!, try not to use to much of the SSD health test softwares. 
That just stresses you SSD unnecessary. 

It would be the same if you would constantly go to a doctor to verify if you healthy and he would give you a xray scan each time, just to tell you you are fine (but to much xray exposure will kill you).