Many people will 'clone' tanks for these same reasons, but I wouldnt want to say exactly how long it would take for your new tank to be ready.
If it was me, I would use a little bit of everything from the established tank to get the new one going. I would put some of the filter media (bio filter parts) in the new tank's filter system. I would also take some of the substrate and put it into the new tank. If you do not want both tanks to have the same substrate, you can fill up a stocking or pantyhose with the established tank's substrate and put it in the new tank. This way you can take it back out.
I would also do a water change on the established tank, but put the water you take out into the new tank. Then fill them both up with new water, being sure to remove chlorine and all that normal type stuff. Finally, I would 'feed' the new tank a bit to get a mini-cycle started, if I can, just to be sure.
The good news is that your mollies are pretty strong and while I wouldnt put them in the new tank right away, you might be able to put a few in a little earlier than usual. Then I would keep an eye on the new tank, testing alot, to see if there is a cycle or mini-cycle started by adding these fish.
So, yea, you can help reduce the time you need in your new tank. It is very common and I have done it with both freshwater and saltwater tanks.
