Just my personal experience. And for the record, I'm not playing to get rich, and I don't absolutely need the money that I make. I'm playing to learn and to improve. I want to play a winning game every time I sit down.
When I am either way up or way down in a session, I find that I stop playing as well. When I'm way down, I tilt a little bit. When I'm way up, I have so much money on the table that I stop taking every dollar of it as seriously as I did when I started.
So I tend to quit when things are either way up or way down. Say 1.5 to 2 buy-ins either way.
If I'm winning and still feeling focused, I may just quit for ten minutes, note the good session in my spreadsheet, get up and walk around the house, scritch the cats, and then get back to work on a new set of tables with new buy-ins. In my head, it's now a new "session." And I only have one buy-in on each table again. For some reason, I play better this way.
Your mileage may vary.