People will have a different number/density of receptors, and also there will be a higher or lower affinity of different growth factors for receptors.
For example, there is a differential response to steroids, some people respond well and don't need to take a lot, some people do not respond as well, need to take more, and there are probably times when even taking more isn't going to make any difference at all.
The point, I don't think that there is any 'set' amount that will trigger gut growth.