Why not just compact based on file size? If the characteristic of a db is that it needs compacting more frequently than some, then surely this is driven by use. Usage could be 3x in one month what it was in the prior month, so does a time period make as much sense as a file size? Not to me. Question is, what is the file size you'd pick? Experience would probably tell you that.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.