I once did a fairly large one (4 feet) late in the season, and was worried about cutting it loose too soon. I decided to wait until the tree dropped its leaves, and then potted it in a larger container more in line with the size of the trunk and branches. It wintered in the garage, and in spring I moved it into full sun; it soon it leafed out and grew with no problems.
I'm finding it's pretty hard to mess up an air layer. As long as the rooting medium doesn't dry out, it seems to work.