Perhaps allow the user to set it in their preferences
This might be better as an idea?
Whilst not the Recent Activity widget, you can control the streams:
And in 6.0 there will be custom streams which might just be for a single place or topic etc.
However I would suggest 'nudging' any group owner or place admin where there is so little traffic that the recent activity widget shows very little over the last 30 days to start driving adoption in their community - and they can always feature older content.