You stop.... when you die. 
So you buy several kits and start the second kit before you finish the first one. After a couple of weeks working on the second kit, you go to the first and finish it. By that time you will be more interested in the second kit so it will be easy to determine when the first is finished. lol
When to say when? For me it is when I get frustrated trying to add details like rope coils on all of the belay pins…ugh! I start a model with a particular general end point in mind…such as Admiralty with no masts, or masts and spars, or sails. The huckleberry is then the level of hull and rigging details I want to fuss with. Most often the urge to move on to that next cool kit overtakes the urge to add more rope coils!