You should start seeing aerial roots develop with in a week or two. They are very tender until they attach themselves to the soil and start developing some bark, so don't touch them except to remove ones you don't want. After they have gotten strong, you can adjust them somewhat next time you repot if you wish.

Check out Jerry Meislik's web page.

Here is a link that might be useful: Jerry Meislik

I agree with bonsaikc with the caveat that "a week or two" applies more to the heat of the summer when everything is growing like crazy. At present you may just get mildew. I did that very thing to a lovely F.rubiginosa I bought in September with lots of fat white aerial roots. I placed the whole thing under a plastic "tent" draped over a frame, my apartment being much drier than the greenhouse whence the plant had come.
In a week or two the fat white roots developed thick fuzz and the smell in the tent was quite bad even though I lifted part of the plasctic daily to let in fresh air. I dismantled the whole thing and saved the tree but the fuzzy roots withered and only the wiry black ones are still there, though I have no idea if they are alive or dead.
In my experience the easiest tree to practice forcing aerial roots on is the Schefflera. It will oblige almost any time.
Do read everything by Jerry Meislik before you do anything, though.
Good luck,
Krys

hello,
I dont know if you want trunk or branch aerial ones. I didnt want to cover my whole tree with plastic so I just took an empty 2 litter bottle of soda and cut the bottom and top off, and wrapped it around my clump of ficus benjamani (which dont even air root as well as microcarpa does)and stappled the two ends of the plastic together. SO this "retaining wall/ring" about 6" high just sat ontop of the soil. I packed the ring with spagnam moss, not peat moss. In home temperatures after a couple weeks I now have roots off the trunk all over the place. The moss really holds moisture for a long time. If you want roots off the branches Ive heard of drapping/wrapping alluminum foil over the limb you like and take the foil all the way to the soil, This will lock in humidity, If you have patience, you may even try to fill this makeshift "aluminum foil tube/lean-to" with spagnam moss.
Castorcrap

i want both