Your Ficus is a tropical. As long as it gets the sunlight, water and tempertures it needs, any time of year is fine for your trunk chop. As to the length of time before you see new shoots, I am not sure (a very few weeks). One caution, with no foliage, its water requirements will be very low, take it easy. Good luck.
Have fun,
Vernon

Since this was posted 2 weeks ago, it may be to late, but ficus benjamina can not be trunk chopped with any guarantee of success. Most tropical plants to do not keep a reserve and can not produce more foliage without some existing foliage.
If you do chop it, you'll more than likely kill it.

I think, given the lovely foliage at the top of the tree, you'd be better off air layering it about 6" below the present branches; that way once the new roots form you've got a nice tree to look at while you contemplate design options.

Here's a link to useful information about this species I found elsewhere on this board:

http://forums2.gardenweb.com/forums/load/houseplt/msg082242071778.html

D.

I have a Ficus Benj that's about 14" tall and there's five trunks to it. Its still forming into a fine upright tree shape and after trimming down the original nine limbs that were in the pot (teacup pot I got it in!) and giving it more foot room, its come back and is doing great. I started another one from cuttings from the huge floor sized plant I have and that one is doing fine as well. Maybe starting from large limbed cuttings would work for you. I used 1/2" dia. limbs to start with and within two months, I had growth and new leaves showing. Good Luck and have fun.