I usually wait until my roots are at least a few inches long, about as thick as pencil lead, and have at least 1-2 branches showing.
After cutting from the parent tree, I clip all but a couple of topmost leaves so the plant can't expire as much water (less water uptake is required). I keep the plants in mostly shade for 1-2 weeks minimum (more sun = more water loss).
I've seen some who like to cut theirs off when you can't even see any soil - totally rootbound. I prefer not to do this. I try to cut mine before the roots cover 25% of the soil pouch.