Given that he's only 100 points ahead of Roger, he will be #1 until he fails to defend one of his clay titles, in order: Monte Carlo, Barcelona, Madrid, the French Open. So it really comes down to whether you think he can repeat what he did last year and if not, at what point does he fail to do so. Rome is unlikely to be a factor as he only has 180 points to defend.
My guess would be Monte Carlo. He'll be coming back rusty and vulnerable to a young and hungry field. Once he loses #1 at MC, he won't have a chance to get it back until Halle at the earliest when Roger has to defend 500 points, and probably not until Wimbledon.