That will depend on him. How much he still wants to play and how much can his body handle professional top football.
That might mean that he can give us 2-3 but maybe even more good seasons. And maybe less than 2
What attracts me in Forlan to Juve is that he can continue doing what Ale was doing. Even if only short-term I see him as a great, realistic, replacement for the qualities Del Piero used to give (yes, used).
Technical, skillful, calm player, who scores ~20 regularly, who assists, with experience, with great set-pieces, whose legs can follow the tempo of today's football.
Some might take it as an insult to compare Ale with Forlan, but right now, I see this guy the closest that gets to Del Piero.