It really just depends on how well your greenscreen shots are keying already... but normally I'd say 1/8 is plenty in most circumstances.
If you are getting a little bit of fringing on the edges, try some 1/8 and see if that helps. I used to use that much in distant years past. Honestly I haven't used any at all in a long time. For the last few years we've switched to Ultra Key (usually in Premiere, although sometimes in After Effects), and it does a perfectly clean job with edges so we've had no need for any magenta. Ultra will also key through smoke, key through glass and preserve reflections, retain shadows on objects, all that jazz.
I would think that if you need any more than 1/8, then you probably have other keying issues that need addressing instead... and if you go more than 1/8 then although you might have cleaner edges you will also more likely have a magenta cast to your talent that you'll have to deal with separately.
Long-winded answer there, just to say "Try 1/8."
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Like you, I haven't used minus green for awhile, but I have a lot of interviews coming up to shoot and key, and for efficiency's sake, I want to give myself every "edge" I can. (Groans, heard from here.)