I accept new members straight away, though as we've discussed before, Peter, I do consider it friendly and "good manners" if they introduce themselves with just a little more than "Hi, I'm new here" or "The rules say I have to post before I can PM someone, so hello."
Often new members are looking for help (if I remember correctly, that's what prompted me to join back in 2010). Once someone has gone through the initial Q&A session with established members, if I see them popping up in subsequent discussions and debates then I tend to assume they may be with us for a little while, or hopefully longer. Their developing demeanour, sense of humour, willingness to participate / listen / share knowledge, and resilience in bouncing back from the occasional heated exchange, are what probably convinces me they could be with us long term. These things also, inevitably, contribute to the impression I form of them, and whether I'm likely to enjoy interacting with them. I suppose that, sub-consciously, I consider members past the newbie stage once I've developed an impression, which usually means I've had a certain amount of active or passive involvement with them.
Although I try to acknowledge all input and responses in a thread, I'm probably guilty of being a little more responsive to those members I've had greater involvement with - be that in a positive or negative way. And there are definitely certain members whose opinions and advice I'm likely to consider as credible - and vice versa!
In both cases, those members tend to be well-established contributors...