The ultimate stage, when you actually decide to quit, is when you believe the effort you put into running is not worth the benefits you get from running.
When it comes to making these decisions, every person is different and unique. Someone whos primary goal, their main or only perceived benefit, is to compete and do well at a national level in the marathon may decide to quit when they believe the effort required to run 120+ miles a week (and do all the extra things) is just not worth it anymore.
Someone running 15 miles a week for weight control may decide to quit when they no longer care about their weight.
You might want to make a list of the benefits or goals you have in running, and also what sort of efforts you believe are necessary to achieve those goals. If your list of benefits and goals is rather short, and/or the effort you believe is necessary to achieve those is great, you will find it easier to make the decision to quit. If you have a lengthy list if goals corresponding with numerous effort levels, it will be harder to quit.
As others have suggested, try focusing on your general basic running goals/benefits, the ones that require less effort to achieve, for a while. (if you have multiple goals). You may decide those are fine and stay at that level, you may decide to ramp things up after a while and chase bigger goals, or you may decide even those basic running benefits are not important and quit entirely.
Some people quit running primarily because they have to (due to injury or a life change situation).
Others quit because they want to. Sometimes its a combination of the two.
After decades of running I quit because a knee injury made it impossible to run without continuous pain. But I missed the routine, the habit, the effort, the weight control, the challenge, being competitive, the joy of being fit enough to easily cover miles of terrain rather quickly with minimal effort, my running friends, etc.
After over a year the knee healed enough to allow me to run some without pain. I started running daily again since I can achieve some of the benefits even while covering far fewer miles at a much slower pace.