My husband and I agree to meet people the other are dating before it gets sexual OR when we think something is going to actually work out. Not that we've had a lot of partners since becoming poly (for the second time) but I'm guessing that would average 6 weeks for me, and 2-3 months for him. Anybody who has stuck around that long has a decent chance of being a long term presence in our life so it seems to be a good time to touch base.
It's different if the person runs in the same circles and its convenient to just say hi before then, but I understand not wanting to be pushed to meet. I have also learned meeting too early can be a problem - once my husband was so excited about somebody after one date that we ended up both meeting the woman for lunch and she got spooked off because she tended to not date married men because they usually had too many "rules". She opted not to meet my husband for further dates because while I was with them he got too nervous about me to flirt with her like he had on their date. When I met him he was a crazy flirt with others in front of me, so I can get why she wasn't interested if he changed that intensely from one date to the next.
I've met potential partners of his, and my bf's wife, both without the men present - for coffee/drink dates. I like this approach in general, if you feel like you're going to be on display for how you behave in front of your partner, it can just be more relaxing to be a one on one with a metamour, and a bit easier to say "well I have to get going" if you get uncomfortable.
Happiness will never come to those who fail to appreciate what they already have.