I want to say Tobi told Naruto that was his plan when he came to see him at/near the Kage Summit, but I'd have to go back and look.

Not only is Tsunade on a time limit, but this is the third time we've seen her use this, and it shortens her lifespan every time she does it, and she's the last of the Sannin left alive. I'm actually starting to get worried about her being in direct combat. I don't really think she has that much time left, based on how quickly Kishimoto's been killing people off, and Naruto's going to need to be the Rokudaime Hokage at some point.