Now I know that blemmy and reefnut have alittle experience on me but still. It depends on what type of skimmer you have "at least to me". The more top of line skimmers usually start working right after plugged in. But the farther down the line you go the the longer it takes to break the skimmer in "till it starts working". I ran my skimmer during the cycle so by the time it was over it was working it took around 2 to 3 weeks before it started pulling anything out.