I have air layered both plum and cherry. It took around 5 weeks to form callus, after 8 or 9 weeks there were roots.
I started mine after picking off the flowers, which is around mid spring.
Plum responded slower, but not by much.
I'm doing a winter cutting experiment as well. But since prunus are notoriously hard to root from cuttings, I'm not advising to copy that.