Ha, ha, I was just thinking about posting regarding this topic. I don't know if I could put a specific time frame on it, just doing the 'poly' thing and communicating openly about expectations and boundaries. I'm assuming you would have your own room/space/suite, whatever makes everyone comfortable.
We are considering having my bf of 7 months move into the basement suite of our duplex and I'm torn if it's too soon. He would have a completely separate suite, but it's still a big step.
I don't know you or Redpepper or Polynerdist, but I've come to respect and admire what I've seen of your lives on the forums. If you're asking for my two cents, I would say you have already a formed a family bond, it makes sense to consider making a home together.