Here's an article that I wrote a few years ago, which talks about the rooting process and about fertilizer:
Creating A Plumeria Collection
It actually takes three months for a root ball to form. If you have the start of leaves coming in, then you more than likely have the start of roots coming in. I would water when the soil looks dry and keep on a warm surface.
Also, in the winter, growth slows down dramatically and even comes to a stop (dormancy) if not given warm conditions and supplemental light. You can give your one that is rooting a bright window with extra light in the form of a desk lamp inside your house to help the rooting process continue. I wouldn't recommend anyone try rooting cuttings in the winter, however, unless you have a heating mat and a greenhouse. It is tough to root cuttings this time of year otherwise.