The photo of a conifer you posted is a spruce. Spruce are not known to air layer, though one never knows until you try. In general for most trees, I would not start an air layer until after the first flush of growth has hardened off, in Ohio, probably middle or late June would be a good time to start air layers.
Maples and most deciduous trees like elms air layer relatively quickly, roots can appear in as little as 8 to 12 weeks.. Air layers are usually left on until you see roots are several inches long, and wrapping around in the bag, or pot that contains the air layer. For deciduous, usually you can separate them in September in Ohio. Some deciduous trees will not air layer for you, at all. But I don't have a comprehensive list. If in doubt, go ahead and try. either it will work, or it won't.
Pines, spruce and many other conifers are notorious for being either slow or impossible to air layer. The few pines I successfully air layered did not produce roots until their second year. Majority of air layers on pines failed. I once left an air layer in place for 4 years on a Japanese white pine, and it still failed. But I have successfully air layered a Japanese black pine, several times, but with only 4 successes out of 10 attempts.
Good luck.
