Hey there,
In my 13.5 gallon I take out about 1/2 of my chaeto each weekend, sometimes stretching it to 10 days or so. With that said, my chaeto is growing too quickly and removing all my nutrients, so i've recently decreased my lighting period.
On my Biocube [with a sump] I removed about 1/2 every 2 weeks, and it would fill back in at around the 8-10 day mark.
On my 75 gallon [with sump] I would remove about 1/3 of my chaeto at the 3 week mark. It grew way slower in this tank [lightly stocked, and heavily skimmed]