IMHO it all depends on the size and weight of the knife you are making the sheath for. I use 7-8 oz. for most of my knives but they are all 8" or under OAL and I try to keep them fairly light. I am making a tactical that will require 9-10 oz. because of its size and weight.
7-8 oz is pretty normal for small to medium size knives.For any thing bigger go to 9-10 oz.You can use 5-6 oz for folder sheaths.FWIW your better off going thicker than thinner.Light flimsy sheaths are not good for protection.Dave2thumbs