There is no end, heres why. ( this may not be popular ) makeup is like painting. Sure you have people that just put it on as they have always done just that, everyday wear. But your into colors, blending, sparkles etc.. etc... it's really just painting. So I would say it's more like a hobby than anything else. You quit or slowdown when you lose interest.
Just look to any hobby, and you'll find groups of people that take that hobby to extreem. Take fish tanks for example. There are people that have saltwater fish tanks. Huge ones in the 400 to 1000 gallon range. They have it loaded with corals, fish, real rocks and sand from the ocean. I even heard about one guy that had such a large tank he had to have a garage door opener installed in his attic to lift the hood of his tank. This is any hobby really. The only limit to your hobby is your personal interest, desire, and pocketbook.