I haven't really looked into the worm tower concept specifically, but if it's the one where you have a tube/bucket with holes in the middle or side of the bed, I thought the idea is that the tower feeds the bed continuously and you just keep adding to it?
I have a separate vermicomposter called can o worms and it's meant to stack the scrap trays on top of each other and harvest the vermicompost from the bottom tray. So I just mindlessly go through the motions and never really took note of the time it takes... But it does seem to depend on the season and temperature.
If you need to, you could always screen out the worms and bigger bits to harvest the finished wormcasting, then put them back. It's not a big deal and they won't mind being disturbed.
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