I grow in pots and I have had this happen. The affect that cutting the roots on them has had has varied from tree to tree. Some show very little sign of distress. Others have to be babied and put in the shade to survive. The biggest change is in the watering schedule.

My best advice is to watch for signs of distress and give them a little water when the leaves start drooping.

The best way to care for your plant is a regular (full) repotting every 2-3 years, which includes bare-rooting, root pruning, and a complete change of soil to one that is durable, well-aerated, and free-draining. Beyond that, you're charged with getting the light as right as you can, making sure all the nutrients the plant uses are in the soil in a favorable ratio, keeping it warm enough in the winter, and keeping it in an o/a condition that ensures it's healthy enough so disease and insects can't get beyond it's natural defenses.

Let me know if you want pictures of a full repot on a F benjamina.

With the roots being as large as you described, a rather large fraction of the plants very important roots will have been attached to those you severed. It's also likely most of the roots in the container were pressed into service as conductive roots, rather than roots that absorb water and the nutrients dissolved in water, so your tree probably took a pretty big hit. How quickly it recovers depends in large part on how conducive to new root growth the conditions in the rhizosphere (root/soil mass) are. You can control that, in part, by your watering habits and by making sure soluble salts don't accumulate, but root congestion will also play a significant role.

Al

hairmetal4ever in the future there are two options. Some people plant their pots in the ground in the Spring and dig them up in the Fall. However if you don't want the roots going into the ground next year you should put the pot on a paver or elevate it with something to keep it off the soil.

The practice of partially burying the pot is much more common and practical where temperate plants are concerned, and not so common with tropicals like Ficus b. It is much less traumatic for the plant if you sever a large fraction of the feeder roots when it's dormant or quiescent (temperate), compared to when the plant is to continue growth and is about to face the most difficult period of the growth cycle (tropical).

Al

I totally agree Al. If you cut the roots from a pot planted in the ground during summer it can be traumatic. I did so five times this summer as I was moving trees to more permanent locations. Some handled it well and others really had to be babied. After doing five I decided to wait for cooler weather. I can't wait for them to go dormant here; some never do. Last year about a third of my trees lost their leaves for two weeks. The others stayed in full leaf right through our winter such as it is.

Here is a link that might be useful: Here is what an average December looks like here.