any one use 135mm (FF equivalent) lens ?

why do you use it? what is it good for ?

how far do you have to step back from the subject?

any thoughts greatly appreciated

For portraits, concert, sports

Once used Canon EF 135mm f/2 L USM paired with EOS 80D. It is very sharp, autofocus is very fast, it is very well built and it is one of the least expensive L lenses Canon makes. Probably one of the reasons why so many people shoot Canon cameras; Nikon simply has nothing that competes with this. All of Nikon's 135mm lenses have goofy autofocus controls.

When you make use of the f/2 aperture with the 135mm focal length, you'll soon find that this lens is best suitable for indoor sports, stage photography and portraits. There are times when it is so sharp that you can see the tiny facial flaws.

For your last question,you'll have to be at least 3 feet (0.9m) from the subject

today's 70200 are really sharp, but in the old days, zoom lens are inferior to prime lens, hence 135 is filling the gap of great tele prime.

135 working distant depends on what composition you are doing. animal? portrait? half body? full body? fullbody with landscape? so there is not rules or a proper distinct working distant.

another sweet thing with 135 prime is the aperture, which is wider then F2.8 (comparable zoom, but recently sigma is coming out with 50-100mmF1.8 tele zoom and rumored 80-150 F2/F1.8)

135mm at F1.8 shooting a fullbody, still gives you very strong seperation(bokeh) oppose to 50mm F1.4 fullbody shot.

another key purpose of 135 is the aperture, like handheld at night shooting, the F1.8 give you lots of light for handheld shooting.(its put it as a recent crop body with iso1600 useable limit. dont bring out camera like A7s or D4s with high iso limit)