I've known people to go 12 weeks fallow and still ran into problems. Take your "new" seperate QT that you're going to setup and when you think your fish are fully QT'd after 4-6 weeks, take the most likely to be susceptible to ich like maybe a powder blue tang and add him to the newly setup system with water from your display that has been 12 weeks (minimum) fallow. Give him and/or any others you think are "ich magnets" and let them reside in there for 4 weeks or so. :::just in case I didn't say it right, do this with your current fish before buying more to put in the new QT:::
As you do that, slowly take down your copper/med levels in your QT that you currently have your fish in. As the level comes down, if they still have ich, it will become apparent after a few weeks or maybe even days.
I know people that have gone thru "near" religious practices of making sure there is no way they could have contaminated their DT, QT(s), RO/DI reservoir & premixed saltwater and still ended up with ich after 12 weeks of running fallow. In the end, it was believed that the fish were clean but ich was still in the DT. Don't hold me to the numbers but I believe @ 8 weeks you're at an 85% chance that you got rid of ich in the fallow DT, @ 10 weeks you have a 95% chance and after 12 weeks you've upped your chances to 99%. Obviously in a QT with meds you can cut that time down significantly; Unfortunately in a DT, patience is best and usually the only remedy.
The hardest part seems to be around the 12th week when people seem to have the idea that it is guaranteed that the issue has been irradicated and they move things a little too quickly into the DT especially when all of the fish in QT show no signs at all for months of going ich-free and we think the coast is clear.
Sounds like Kathy, Sunny Mbg, & Russ are offering the best advice in terms of going 2-3 times the amount of time to run fallow. It's not worth the 8 weeks to gamble and possibly start all over again especially when you're thinking about bringing other fish into the mix.
Best of luck and I hope it all works out for you!