Water roots and air roots are comprised of different tissues.
Often, water roots will break when potted in soil, which means that cuttings
will be rooting twice. For this reason, I root in soil directly...or else pot rooted
cuttings in soil as soon as I see evidence of root primordia on the stems.

In other words, I'd pot them now. The sooner the better.


Josh

I agree with Josh, rooting directly in a pot with your favorite growing mix is often the best method. Having said that we are talking about Hoya multiflora or javanica and I was unable to root cuttings of my plant easily until someone here mentioned water rooting it.
I have some cuttings of my plant at about the same stage as you describe, roots about 4-5" long. I am planning on potting those cuttings up this week. The roots look much sturdier than the were even a week or two ago so I am hoping that they make the transition smoothly. Keeping the newly potted cuttings warm and moist couldn't hurt either until they settle in.

Mike