I don't see Marvel writing out either character pre-maturely. I figure Strange 3 will hit before those two Avenger movies in phase 6 and we'll see Strange become Sorcerer Supreme there, in order to get him to his "final form" in time to deal with Secret Wars and Kang. Where that'll leave Wong is anyone's guess but the character is a long-standing member of Strange's supporting cast and has proven quite popular in phase 4, so I expect he'll be there for Kang too. Marvel won't want to lose Wong just so Steven can have the title, especially when *not* being Supreme doesn't seem to be slowing him down any.
Marvel likely enjoys having two popular wizards to throw around the MCU and won't want to get rid of either of them any sooner than they have to. So the real question is how long do the actors want to stay in the roles?