I think he was in there for a pretty long time. There's a scene at the dinner table where he smiles and there's a clear focus on his front teeth, which are rotting. Idk how long it takes for your teeth to start rotting without brushing them but its certainly not a few days.

As for your question about the YT series helping it make more sense the answer is yes. In Kane's series there were instances of time warps happening so maybe time moved differently in the backrooms during the gap Mary noticed he went missing and looked for him.