Well, how many fish are going in it? Saying that, it doesn't really matter except that you can’t over crowd the hospital tank just like you can’t overcrowd the main tank. Waiting a few days is not going to matter because the tank is new anyway, even with the water and live rock it is not going to complete a cycle in just a few days. The minute you load it with fish, it is going to potentially get thrown into a cycle. You will have to make sure that that doesn't happen.
You are treating ick, correct? In which case, you can not leave the LR in the tank once you begin to lower the salinity in the tank. In order to prevent a cycle, be prepared with lots of well-mixed salt water to do water changes every day. Also be prepared to siphon off any uneaten food that settles on the bottom of the tank [again every single day]. The longer you wait to begin treatment, the less chance your fish have of surviving the disease. They can succumb to bacterial infections in addition to the parasites just because their immune system is compromised.
Thus, begin the treatment, and don’t allow your tank to cycle. Water quality will be just as important as the treatment itself. Be sure to ready your ammonia and nitrite test kits, and test daily. Don’t wait for signs of a cycle to do a water change. Change out at least 2 gals a day. If you want, you might be able to find a bacteria in a bottle product at the LFS, and you can use this to push the age of your tank a bit. If you use this, however, be sure to get a product that has an expiration date on it, one that has not expired!
In lieu of LR you will have to place comfort zones in your hospital for your fish, whether this be PVC pieces or fake rocks. This, too, is important. BTW: Did you paint the bottom of your tank black? What are you doing for the bottom of the tank?
