What you'll be told:
6 to 8 weeks
The actual amount of time:
That varies. Depending on a feature request, it's something that could be very easy (like say, fix pluralization bugs, or maybe not) or very difficult, like say putting completely new subsystems in.
And that's assuming the feature request is even being considered. Not every feature request has merit, and even of the many that do, sometimes the work that is going to be put into implementing a feature on the site isn't doesn't justify the work behind that feature.
Stack Exchange is a company like any other: they have many things that they have to prioritize; one feature request has to be balanced against time, money, resources (people) other feature requests (of which there can be hundreds, if not thousands), bugs, new features, maintenance, etc.
In other words, I wouldn't expect a hard number on it. The best you could do for a feature request is to hope that it will be tagged status-planned which indicates that it's been prioritized by the staff and that it will eventually come (if not already being worked on).
If you really want to increase the likelihood that a proposed feature request is considered by one of the members of the Stack Exchange team, you should start a bounty on that question.