Can someone give me a time frame on how long a air layer takes to grow roots on a Japanese maple? I started one about five weeks ago and I don't see any signs of growth.

To be on the safe side wait until autumn. You can't pot the branch in the summer anyway.

In some I have been able to separate the layer after some 3-4 weeks. In others, especially the red-leaved varieties, I have had to wait much longer (One of them took > 12 months)

A ood sign with maples is often a strong increase of branches below the layer site: I find the plant is then in the process of growing new roots. Make sure the layer site gets lots of sun (=warm) and the plant itself is growing strong. This helps the roots to form.

Note that you should avoid separating the layer if you do not have 4-6 weeks of growing seasons left in the year. It is then often better to separate the layer next spring.

How do you know it is not growing roots? It is not a good idea to open the wrapping; Rather, just wait untill you see roots appear on the inside of the plastic.