I'll talk about me, and only me.

It seems once I got accustomed to teh herb (ceased to be a rookie, I guess) I found that a certain ammount of herb would always do the trick, less would not, more would be unnecessary. True, there are factors that affect this ammount, but in general you can take them into account (How long ago did I smoke last? How tired am I? And so on...), so I would say yes a predictable and widely variable tolerance would tend to develope on the individual basis.