Meta is used to decide site policy how should we use the results of questions? Is the highest voted answer immediately taken as a rule or is it a suggestion? If it is a rule then what consensus is needed to set a new rule? Would a vote of 1 on an answer be enough or does there have to be a large number of up votes? How large is that number?
It depends, and it's necessarily vague.
Suppose a site's community, via meta, voted overwhelmingly to, say, outlaw question closure. That would be interesting, but it wouldn't change anything. At the other end of the spectrum, if a site's community votes to change the name of its chat room, or to define a tag synonym, or to reopen a question, there's probably no reason not to do that.
A community's custodians, including its moderators, cannot have their hands tied by a meta consensus if there are reasons to not implement it. But, by the same token, it would be unwise to ignore that meta consensus -- something is driving it, and chances are there's an underlying issue that we need to address. So maybe it's not even about the specific proposal; perhaps that's a symptom of something else and we need to talk about that.
Sometimes meta is data not democracy. Other times it's democracy, and if there's clear consensus and no objection on a site-internal matter, then it'll probably happen when meta activity has died down (people have had a chance to object) and somebody gets around to doing whatever the thing is (merge those tags, update the sandbox rules, start the monthly challenges, whatever).
I can't give you a set of rules. I think our community has been pretty good at both reaching consensus and reaching consensus that we've reached consensus. If you see something that seems to be languishing but also seems to have consensus, bring it up somehow -- comment on the post, bring it up in chat, etc.
As Monica already said, it depends. I've certainly seen it go both ways on Meta Stack Overflow and Meta Stack Exchange...
Some proposals are incredibly popular, yet never really get full support or implementation, while others seem to make the grade and are eventually added as policy.
One workflow that seems to work well for a lot of issues and one that I support would look something like this:
Ask a general discussion question to see if the community recognizes that this is an issue that needs addressing.
If it looks like people generally agree that there's a problem ask a question about potential solutions.
If the feature gets implemented or the faq added then you have solid confirmed consensus.
It's a little harder to gauge community consensus here than it is on some of the larger sites where you tend to see a little more Meta traffic... I think we should be careful to keep in mind that some of our higher voted posts here on Worldbuilding's meta only tend to see a handful of votes and may not always be fully representative of the larger community. Honestly I think we could do more to encourage people to get involved in the process, but even then a slow steady work flow should keeps things on track.
Perhaps adding faq-proposed to the featured on Meta list could draw some more eyes before we move forward with new FAQs?
What if the highest voted answer has 3 votes?
I think there should be some sort of minimum (overall) participation before a policy decision is made and perhaps some special tag for "imminent policy change" that we can monitor.
When there is such a policy change vote on the board, one of the answers could be "ABSTAIN", so we can at least see people have seen the imminent policy change and don't care -- thus knowing we have reached a sufficient level of exposure to conclude that a consensus has been reached.