I realize this is an old question, but it needs a proper answer.
Plumeria is highly susceptible to root rot and will die if you attempt to grow it without potting it in soil. In order for you to have had what success you have apparently had, the cutting itself would have had to have been sanitized (usually required by customs laws when transporting plants internationally) and your current environment would have to be relatively free of fungal agents such as active fungal growth or spores anywhere nearby.
Generally, even in these circumstances, Plumeria will still develop root rot and eventually stem rot and die. You should check your plant for stem rot by pressing with your fingers around the stem from roots to tip; the entire stem should be very firm to the touch with no give or softness at all.
Even if you are miraculously successful at continuing to grow the cutting without planting it, it will outgrow its container within a couple months. When properly taken care of, Plumeria grows quickly and it can grow big - 3-4 meters tall and several meters spread; it's an actual tree, not a house plant.
I recommend that you plant the cutting immediately in a well-drained non-acidic non-saline soil. A 50/50 mix of potting mix and perlite is a good starting point. Pot size depends upon how big you're willing to let it grow. As it grows, you'll want to keep a close eye on it for pests (I have particular trouble with spider mites) and disease. When it starts to get too big, you can take a cutting and root it and start another plant. You can also cut off the ends of a stem or branch to induce branching. You should fertilize it every so often, especially if you want it to flower. Plumeria is full sun and you'll want as much extra light as you can put on it. If you keep it well lit in a warm environment it will continue to grow year round. You probably aren't the only person in your area trying to grow one, so look online for local Plumeria garden clubs for tips specific to your locale.
Plumeria is a very pretty plant, but it takes a lot of attention and care.