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.