Kelley Technical's Zeron can last up to 6 years if done right, but don't count on more than 5. Check out their site:
http://www.kelleytech.com/
for what is, and isn't possible.

I'm sure that some of the other paints are OK, but I know that some of them are not, and I don't have reliable info on which are which . . . so I'd recommend Kelley.

Do NOT let a contractor apply it thinly to save money and make application easier: YOU buy the paint and make SURE it all goes on the pool! Doheny's online has had fairly good prices.