You're done with the hifi treadmill when you stop looking for what's next. You might be open to stumbling on something but the next upgrade is no longer a quest. The new thing didn't make you any happier. I don't need a better speaker than Zu Definition 6; I don't need better amplification than Absolare Altius 845; I don't need a better DAC than a Bricasti M21 Platinum; I don't need a better phono cartridge than Ortofon SPU; I don't need a better turntable than a modded Luxman PD-444; I don't need better tonearms than the six I have; I don't need a better phono pre than a Nagra. Unless something breaks, I'm not looking. I don't need better anything anymore to enjoy music as much as ever.
You might also be done when something deeply disruptive upends your life. Other developments can, without warning, render hifi a trivial pursuit even if it makes music even more essential.
Every gear-oriented interest comes with the same problem. First world problems. When is a car fast, quick or agile enough? When are the guitars you have enough to satisfy? When does it stop being satisfying to spend more money on watches? When is a camera the last one? When does an amateur astronomer decide optics are perfect enough or they must draw a line against striving for more aperture? In the moment these pursuits all make sense somehow, but if your life gets struck by a meteor, you'd listen to music and care a lot less what you play it through. You're done when something elemental changes your perspective & priorities, or it no longer makes financial sense to keep running the game.
Phil