Where is the limit on duoQ and eloboost?

The title says it all. I was just wondering where is the limit and what is considered a normal duoQ or eloboosting, since I used to play with a friend when we were both in low Silver, but since he stopped to play, I reached Gold IV, and he came back, but he's still Silver III. So I was just wondering what would be the limit and if I can duoQ with him freely or not.
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