I wouldn't be surprised if he retires after 2015, 2016 or 2017.
If he pulled off the triple, or maybe even the triple in 2015 I could see him retiring on the final podium right after they hand him the mic. But I also wouldn't be surprised to see him sign a one or two year deal. Maybe he wants the record for most Vueltas, maybe he wants to target the 2017 Giro (100th edition, I believe?), maybe there are some week long stage races he wants on his palmares, maybe he will give it his all in some of classics he has some potential in? If he is interested in any of that I see him staying for 2016-2017. But 2018, he would be 35 years old for the season and I can't imagine him wanting to race then. Of course he could surprise us.
Side note,
Contador riding the Giro is proof that dreams come true. 