Potential SPOILER for the final act of Avengers: Age of Ultron, so if you don’t want the ending spoiled for you, you should probably stop reading now.
According to HitFix’s report, (Just remember it's not confirmed) which follows a visit to the set of Avengers: Age of Ultron, Ruffalo’s statements about there being no Planet Hulk movie are technically true. In fact, there will likely be no Hulk solo movie at all.
At the end of Avengers: Age of Ultron, Ultron is forced back into the physical world, where Hulk can do some smashing. Seeing that Ultron is an A.I., the Avengers decide that the safest way to dispose of him is to send him into space via Quinjet, but the Hulk is inside with him. Banner ends up stuck on a Quinjet set to autopilot with no way to turn the thing around. It’s basically the beginning of Planet Hulk, except that the Avengers might come out with their hands clean because this was an unintended side effect of defeating Ultron rather than part of a premeditated plan (presumably, we’ll see how Joss Whedon plays this one).
From here on out is pure speculation, but the HitFix report presumes that Hulk will crash land on a planet in the vicinity of the Guardians of the Galaxy. Guardians of the Galaxy 2 will then co-star Hulk. Unlike in Planet Hulk, Hulk won’t be stuck on a harsh gladiator planet, meaning that Ruffalo’s Banner can get the screen time Ruffalo has said he wanted for the character. The basic plot of the film will be the Guardians trying to bring Banner back to Earth, an interesting homecoming for Star-Lord as well.
Presumably, the Guardians’ trek to Earth turns Thanos’ attention back towards the planet, setting up Avengers 3.
Again, none of this is confirmed and that last bit is conjecture at best, though it does seem to hang together pretty well.
Avengers: Age of Ultron opens May 1, 2015.