By the time you quit, it's probably long after you should have. By the time you're actually even thinking about quitting, it's probably already past the time that you ought to have quit.
Quitting late is worse than quitting early
Craft → Decision Making
I think the time to quit is when you can no longer see the vision and you can no longer really believe the vision. And then when that happens for a long period of time... or maybe you're no longer even excited about the vision.
For everything in your life, ask yourself, 'Knowing what you know today, would you do something different?' If the answer is yes, then do something different today. Today is the first day of the rest of your life.
The core idea is ask for what you want. Turns out when you actually ask for what you want out loud, you're much more likely to get it.
I like to say the only thing that's worse than a channel or a tactic that you tried not working is when you didn't give it the appropriate shot and you pretty much surely or erroneously concluded that it doesn't work.