I'll spend a lot of time "tweaking" when I'm doing edits, comps and basic pre-mixing, and I stop when I think it needs no more changes and it's just waiting for the final mix.
Then I walk away from it for maybe a week or two and work on something else that's at some other stage of the production...just to get the other song out of my head from the time I spent on it during the edit/comp/pre-mix stage.
When I decide to do the final mix....I'll set aside a weekend, spend one day getting all the processing/
effects fine tuned, do a few rough mixes with some different setting, and then stop. The next day I'll listen to those mixes, make some adjustments based on what I'm hearing, and then I'll run maybe a half dozen mixes, doing the whole vocal up/down, lead up/down stuff....and I'm done. If I still not sure, I may leave it alone for a few days or the week, and do another half-dozen passes, and pick the few I like best.
Since I mix OTB from the DAW, I use a lot of rack gear and my console....so all the gear settings and patching is can't be saved in some file. I do write down settings and even take a few pics of the console settings....but basically, it's not something I want to have to keep coming back to week after week after week and resetting...plus I want to move on to something else, and I need to zero the console/gear to do that...so that helps me to NOT want to tinker endlessly and I've learned to finish a mix and move on. I've thought about going back and completely redoing some mixes....at some point...but I've yet to do that, and there's some I certainly would like to have done differently, buy I just rather move on to a new song than keep beating on something I already did.
I can see how ITB allows for endless/repeatable tinkering...so you have to just make a decision to stop at some point.