Every day we get people in the shop who say: I have no more room to collect anything. To which I should (but don't) say: You are not a real collector. Real collectors cannot resist the thrill of the chase. There is always something new or different or rare.
Years ago, I stayed at a bed and breakfast in historic Williamsburg, Virginia. It was one of those grand gracious southern colonial houses with a broad staircase and lots of rooms with crown molding. The people who owned it were retired and they hosted guests just for something to do. They also were collectors. They had china and glass and baskets and Mexican pots and inkwells. They had a complete collection of Hummel figurines. In fact, they had several "complete" collections. And that raises the question: when is a collection complete?
If your aim is to collect one example of a finite range of things, that might be attainable. If you decide to extend the range or upgrade the examples, your collection will continue to be a work in progress. And I guarantee that whatever your collection, it will spark an interest in something else that is collectible. We have a resident here in Old Town who has amassed several "complete" collections, donated them to various museums, and started all over again on something else.
So when do you stop? If you are a real collector, the answer is: Never.