I'd say it's "too much" if the existence of the quantity oppresses/depresses the person - e.g. can't go into the sewing room because of feeling overwhelmed.
I'd say it's an "addiction" if you cannot control your spending. We've all over-spent at one time or another - and probably more than once - but if it is happening on a regular basis, making the person unhappy, and they can't stop then it is an addiction.
I have zero medical training, so I can't speak from that sort of informed viewpoint, and I've never been diagnosed with an addiction so can't speak from knowledge of that sort of standpoint either, but I would say that when the thing is controlling the person rather than the person controlling the thing then that is addiction.
I know that I have bursts when I will be intensely into quilting - the actual doing - and then I kind of burn-out and have a lull. Probably biased, but I view that as (fairly) normal. LOL
I don't do the same thing with spending - there's usually an identifiable reason for an overspend, e.g. visiting a quilt show - but I can easily imagine doing that (esp with the lure of the Internet) if the money was available. It isn't, so I don't, and I think that is part of my definition of 'healthy enthusiasm' rather than 'addiction'.
Of course there will be other variables, such as if a person is so consumed by the quilting/shopping that they neglect to spend any time with family/friends unconnected to quilting, etc.
Just my totally-off-the-top-of-my-head opinion.
:-)