There should always be some DOC
's, because you got food not being eaten, and fish doing the doo. But as your tank gets more established, and LR
develops, the amount that your skimmer removes will go down.
When I first set up my 55g tank, I was getting a cup full ever few days, after 2 years, I was bairly getting a cup full a month.
Now I just set up my new tank, 90G, and I've been getting a cup every 4-5 days.
Alot depends on your bio load, how much you feed vs how much gets eaten, what you feed, etc.