This editing continues until the painting is finished. The criterion for what constitutes a “finished” work is reaching the stage at which you are no longer sure whether applying additional changes makes it better or worse. So there is a real possibility of making things worse than they were by not stopping at the right moment. Incidentally, this is the main argument for taking frequent breaks from your work, even at the risk of interrupting a flow state. Doing so allows you to take a more detached, if not completely objective, look at the current state of your work and thus avoid making costly mistakes.~ Peter Oshkai from, http://peteroshkai.com/2020/06/04/ways-to-fail/
This is a brilliant way to tell when to stop.
I also believe that stopping while in the flow state is a good way to set oneself up for the next working session. I call it “parking on the hill”—which is a reference to strategically parking one’s car, nose downhill, on a hill so that it can be jump-started using the manual transmission. When stopping a work session, it’s obvious how to pre-position all the physical materials, the space, etc.. But stopping mid-flow also means, in my opinion, your mind is “parked on a hill” as well.