Ficus Benjamina root growth rate
My tree started out in an 8" planter and was 4' tall when I got it last winter. Moved it up to a 10" right away, then 12" at the beginning of summer and now to a 16" about a month ago. I pulled up on the trunk today, and to my surprise, the entire soil mass came out intact and the entire bottom and outside were completely colonized with feeder roots. It's taken over about 4" depth of soil and 2" around in just one month! Do these normally grow so fast? I feel like I'm going to be in such a large pot in a couple years that I won't even be able to move the tree. I'd like to get it to about 7' tall (currently a little under 6') and then keep its height pruned. When I start this, should I root prune too? Now I see why they say not to plant these outside around a house in Florida. The roots take over much faster than I ever expected.
There is no way to predict the growth rate of ficus or generally any plant material.
Environmental conditions and care vary and effect growth.
Since your so diligent and giving your Ficus such good care, it's no surprise that your plants appear healthy and are growing well.
I really would not suggest pruning roots unless they appear damaged or diseased.