I started running my skimmer right away. It took about a week for it to stabilize (it was foaming like crazy and overflowing, I just let it overflow back into the sump), granted it wasn't actually "doing anything" for the tank, but it helped break the skimmer in. I would recommend from my recent setup experience to run it and let it drain back into the sump, then it won't mess with the cycle and will be ready to go when you want it to start pulling stuff out.