for me its when its not fun anymore.
I took a long break out of the hobby a year ago, and when I came back, it was fun again. sometimes that's exactly what you need, a break. As for my main PC of Martin brodeur..... well the costs of a lot of his cards are way out of my reach at times, the number of cards itself are too many, so i just collect as much as I can base/parallel/insert wise when I can, and get a high end piece when my budget alows me too.
i'm a super collector by no means, but i'm ok with that. I like the mini milestones along the way. for example im about 10-15 cards away from hitting the 100 card mark for my Brodeur PC, and that will be one awesome day 
