Cycling 20 long

So I'm starting up a 20 Long mostly for fragging. I have put a few pieces of live rock in my display tank to seed it and will transfer that over to the 20 in the next few days. I also ran a power filter over my display tank for months and put that on my 20 so hopefully the cycle shouldn't take long.
When do you think I should start putting in the rock from the display tank? It was dead before I put it in so how long will it take to seed.