Well, if I read Smith's appearances on The Late, Late Show correctly, he picked up an American girlfriend while shooting in the U.S. Consequently, he's spending time on this side of the pond when he can.
Naturally, if the relationship keeps going, he's going to start thinking about trying his luck in Hollywood, probably sooner rather than later to be closer to his GF.
No doubt he'll keep whatever his contractual commitment may be for Doctor Who. However, I'd watch the GF. That'll tell you more about when he plans to go to the U.S. permanently than anything else.
Much as I dislike wishing anyone an unhappy love affair, if he's to stay beyond his current contract, that's what one would wish for.
Dakota Smith