"Stopping is even more difficult and important than starting, as the massive literature on problems of impulse control shows all too well. ...
... So long as writers cannot hold back and stop on time, they do not become productive and healthful workers. Why, exactly? When they fail to hold back from busyness and avoidance, writers rarely end writing before diminishing returns set in, and so make the work aversive and the writing superficial. And so long as they run overtime at writing, other important things get put off and eventually supplant writing.
.... When we cannot stop because of the short-term rewards of continuing, we binge. And when we binge, we usually get one major task done per day. That often mean that the next day or two must be devoted to work other than writing, or maybe a needed rest.
You might recognize the problem more readily among the teachers we saw earlier (Section I) who hadn't learned timely stopping. They waited too long to set off for class. They rushed to the podium and lectures at a rapid pace that left many students detached from the class. And, by running their classes past the bell, they tired themselves and their audiences. They got less from more." (p. 145)
My take on this: To stop timely you have to develop a trust in yourself!
This is also related to an older post, claiming that the unit of presence is about 15 minutes!