Some recommend a minimum of 6 GM/day. I just take a little along with my glucosamine/chondroitin mix (joints). I've taken up to 8 to 10 gm a day in the past w/ no problems.
Some will use higher doses for allergy symptoms. I know a doc who uses it IV for many conditions, things he used to use IV DMSO for (pain states, fibromyalgia , etc).

My ElastiJoint has I believe 1.5g MSM in each scoop and I take that twice a day. But it also has Glucosamine, Chondroitin and Collagen in it too.
Works for me!

I personally take 16 grams a day. I started with 6 and found that it was more effective up to this current dosage. I have been taking it for ~4 1/2 years now (everyday).