I am no expert but here are a few things to consider to help you understand. Take into consideration the use of what you will airlayer, your geographic location, and the health of your tree. Lets start with the health of your tree, is the tree healthy? Did you fertilize heavy last year to prepare for this? Is the tree recovered from any other work that you may have done to it? Next we will look at geographics. If you are in Connecticut your growing season will be shorter than mine here in South Carolina. You need to consider the time needed to allow the tree to generate roots. Differant species make roots at differant rates. And lastly what part or parts will you use after the airlayer? Lets review plant biology, plants use sunlight and water (really sugar) to make starches. They mostly do this in the leaves. The starches are then sent to the roots where most is used to feed (grow) roots and a small amount is turned back into sugar and sent back up to the leaves where the cycle is repeated. Air layering is posible because you stop the starches on thier way back to the roots, they form a callus and if you keep them moist they will create roots. So... If you are going to discard the old(original) root section and you have a short growing season and your species is a slow grower of roots then you will want to airlayer in February to get the most growing in a season. If you are going to re-purpose the original root stock and you live in Florida and you are airlayering a ficus you can probably start September 1st and take it off on September 15th. Well..maybe that's stretching it a little. I have airlayered about two dozen different species and about 200 plants in the last 25 years in several locations (mostly southern U.S.). This is the best advice I can think of if you need help on a Tree or process: list a specific species, list a specific process, and if you have the tree handy show a picture and describe what you want to achieve. The knowledge and experience on this site is really incredible! Hope that helps. Happy Bonsai-ing, Don.