I feel the threshold is somewhere along when there are a number of things in your arsenal you don’t use often. I was one of the many here who was in search of the ever elusive “perfect setup” for quite sometime.
Once I embraced the futility of that notion I slowed my purchasing habit a great deal and also became much more productive. I don’t have guilt about the gear I have to pull out to use as long as it is happening semi frequently. It’s the stuff I don’t use anymore at all that is more bothersome. This is one of the reasons I have started off selling some gear.
The other is overlapping function. I now have enough synths, sampler, and drum machines anyone should ever need. So now my purchase habits are to fill other gaps or rolls.
Now I have two self imposed rules for my modest home studio. Rule one, there is a strict one in one out policy. If I want a new synth, sampler, or drum machine one has to go. Rule two, the new incoming piece has to do something new that my other gear doesn’t do as well or at all.