I'm not OC but I think I can answer this.
I use a QT for my 125 gallon because of the time and money spent on it. So when I get a new fish I put him in my 10 gallon QT. I then crank up the heat to 86. I also treat with an internal parasite med as well. Then after a month of the fish being visibly healthy I add him to the 125g.
OC is right. Ich is a contracted parasite. It does not lie dormant waiting for the right moment to strike. It can't survive that way. It is a parasite and must always be feeding and reproducing. It is possible to have it present and not see the white spots that are generally associated with it, but that means your fish have sufficient slime coats to repell the parasite. However it will in such cases live in the gills. And the symptom of that will be your fish violently rubbing against objects, aka flashing.
If you leave no fish in that tank for ten days, the parasite will die and you will not have ich anymore. If you understand the life cycle of the parasite, you understand how to never get it. Based on my personal experience, any fish from the chains, you have a 50/50 shot of getting ich. Two weeks ago I bought a few fish, one from my trusted cichlid supplier, and three from Petsmart (glo barbs). My cichlid that was in QT does not have ich. The glo barbs that I did not QT and put right into my sons' 30gallon brought ich with them and now the whole tank has it (which I am currently heat treating).
This article was referred to me back when I had run ins with ich. It offers a great analysis of the parasite.
Ich | The Skeptical Aquarist