I have had very good results when in full growth in spring. The one in the picture below was started on 1 May and took two months to fill the bag with roots. I could have separated it in much less than two months. You could also start the layer earlier than May, as long as the tree is in spring growth.
Have had air layer in place since end of Mar on pomegranite. Using green globes. Can't see any roots yet but Temps are seasonally soaring. 97 today. Too soon to open the globe and have closer look?