If you mean when Ducks went to prison for life after the Commission Case, Stevie didn't have his button yet. But once he got made he made so much money with the DiNapolis in the construction business that he shot up very fast.

If you mean when Ducks died in in the early 2000s, everyone was deferring to Stevie by that point. After he did his own bit and came home himself, he was for all intents and purposes the Boss by then.

And before anyone mentions FBI charts or whatever, I'm not saying that it was official. But here, in New York, ON THE STREET, WHERE IT MATTERS, he was already running things.