Some, such as Black Madeira have been tough year in and year out. Others have good seasons and not so good seasons. It depends on the quality of the cuttings (size, how long stored, and under what conditions, etc), consistency of your rooting process, temperature, humidity, fresh air.
As Jason said, take a dozen cuttings from the same tree, at the same time, and treat them the same, and sometimes they all root, sometimes some root, and sometimes a batch (for whatever reason) just doesn't work.
I air-layered several varieties this season, and most took 4-6 weeks. Black Madeira is doing it, but at 8-10 weeks they are not as far along as others were at 4-6 weeks.
There are so many variables, that it is always hard to discern exactly what the reasons for success OR failure were.
When I sell cuttings in February, I take them as close as possible to the first shipping date (last year most were less than a week), refrigerate them immediately and only offer them for about 8 weeks. After that I feel that the quality drops off, so that I can't be sure that they are a product of peak quality. But I have rooted some cuttings after more than 6 months in storage.
It is still an art, not a science.