When you are broke?
When you are too busy to make up work for yourself to do?
When you realize that making this or that more shiny won't alter your enjoyment or how often you play a given game?
Considering people send NIB games to HEP for "restoration" there really is no limit.
In my case, projects like re-doing my Flash Gordon Cabinet would make it look "better", but is not going to alter how much I enjoy the game at all. I would have 15-20 more hours of my life into making the game "better" (I already brought it back from the dead and did a budget playfield swap in that game's case).