As for lifetime, I have read that some cells last less than 3 years while others, such as myself, have lasted over 7 years. But I think a lot of it comes down to pool water chemistry and run time.

Most cells are rated in amp-hours. So putting a small cell on a large pool can reduce the life of the cell because it needs to run longer. But of course smaller cells are less expensive too but usually not enough to compensate for the shorter life.

I believe that chemistry also plays a role. I have been successful managing CSI so it doesn't get too high and cause scaling on the cell. Cell scaling requires cleaning with acid which I believe has the potential to reduce cell life if done frequently.