Just play until you don’t feel like it anymore. If you need a milestone, clear a match/run/whatever on the highest difficulty. Or maybe every variety, so in Civ, a victory of each type. Or if you feel like going hard, find a rating you want to get and get there, like Celestial in Strive. I would say just play until you feel satisfied though. Don’t worry about what others think of your completion metric. None will satisfy everyone.