IMO the minute you are hired as an at-will employee that can be terminated at any time, with or without cause, you have been granted carte blanche to do the same to your employer.

That said, staying for ~2 years and jumping when you can get a promotion by doing so seems to be a pretty common strategy. I've heard hiring managers and engineers decry this practice repeatedly, but I know way too many directors and VPs who got there by doing this very thing to feel one should criticize it. Or, sigh, hate the game, don't hate the player.