I think you're right about the timeline. If it were going to happen soon, we'd be seeing referenda and ballot initiatives already.

While the feds may not lead off with an attack on a state attempting to leave, they also wouldn't just sit by and watch. Would you be surprised if the CIA tried to overthrow the leadership of the new government or meddle in the election or arrest separatists under domestic terror statutes? It's a situation that could escalate pretty easily.