If it's working in the complex way I suggested that would actually be quite good. It would let older topics drop off the top 5 list, unless there was continuing interest in them, which would keep the list fresh with its virtual finger on the virtual pulse.
I was just about to say the same thing. The 30 days was the default setting. We're going to shorten this somewhat, we're going to experiment with having it refresh 24 hours in the near future, see how going from one extreme to the other affects things.
24 hours might be too short, as this would be much more likely to bring response posts into the top 5, rather than the original post of the thread, which means you could end up with multiple posts from the same thread in the top 5. For example, right now the 'update long overdue' thread has two entries in the top 5 for both the last 24 hours and the last 7 days: