time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
I don't like it.
It embodies the assumption that any post above once per day (averaged over two weeks) is not contributing; which I do not agree with.
At the very least number of posts per day should be increased, e.g. min(time * 28, posts).
And you should consider more sophisticated metrics such as sum of f(x) over all two-week periods, f(x) being x / (1 + sqrt(x/14)) or x / (1 + x/28).
Time spent logged in will also be useful to include as a component.
I flunked math, but I could add that logged in time is easily gamable now with sites like newbiejail.org, so it would probably make more sense to judge length of posts as well. The best thing would be if Theymos accepted my request months ago to make a "post vote" and "user trust circle" function to allow for ratings of posts by content, eliminate newbie jail entirely by having a trusted user trust you, etc.