I think Sessions is there till the end of 2020. I think Cruz, will be appointed into the position, and hold it until some seat in the Supreme Court comes up...to be appointed there.
The card that Trump holds....is that he could appoint another second special prosecutor on his own without Sessions involved, with his own instructions to limit the focus of the individual. The Senate (particularly the GOP crowd) can’t do much about that, and it’ll keep the focus on the FBI/DOJ players, and maybe even the Hillary email server episode.
We’ll see.