That's very difficult to say, and depends on what happens with every new day. In the past, some have been surprised when upgrade suddenly went very junior. Right now, I don't see anything on the horizon that would cause this to happen again.
Brasilias are gradually being retired, and every one that goes away means a few less EMB crews are needed. This cancels out a portion of the growth. Since all the growth has been jets, the majority of upgrades have been in the jet, and they're going fairly senior. There have even been a few junior Brasilia captains who put bids in for downgrade to RJ right seat because they can make almost as much money AND have a life (i.e. hold a line).
My own guess given what I know as of TODAY: 2 to 4 years