I start either right before bud-break, or when the leaves harden off. Sometimes at bud break the tree has too much going on. I have been successful doing it both ways. As far as how long before you can separate, that is dependent upon the species. I have a tagged Japanese Maple "Atropurpureum" which is nothing more than a supposed red leaved seedling. That one didn't sprout any roots in a year. I checked several times and it looked good with signs of callusing and possible roots, so I told myself "Alright!, 4-6 weeks and I can separate!". Never did put out any roots but it hasn't died yet. I have a Glowing Embers Japanese maple, which I have read is a clonal variety, meaning that it is propagated by striking cuttings and keeping it on its own roots. That one and a standard green JM both put out roots and could be separated in 6-8weeks. If it is a green leaved variety, non-dissectum(cutleaf), I would say that you should have the same results. If it is a cut leaf, I have heard they can be problematic getting them to throw roots. But I don't have any experience air layering cut-leaf. I am going to be doing some air layers on my newly acquired Shishigashira soon. Good luck and just try not to disturb the bag until you can see root tips poking out of the moss. If you get to the 10-12 week mark and don't see anything I would remove the bag and see what is going on, chances are you may not have cut deep enough. I am sure others with more exp air layering will chime in. I like to also slip a pot around the roots, sphagnum and all, after the bag is nice and full of roots. In the picture I put a pot around this one for 4 weeks after I had a good amount of roots, then separated at 12 weeks mark. But that is my personal preference. As far as your application, looked textbook to me. Good Luck!
P.S Might as well get a few going on that tree while you are at it.
