Marketing guru Seth Godin offers a little book (seriously, it’s only 76 pages) that tells you when to quit. The Dip is all about quitting what doesn’t work and sticking to what does.
The idea is that if you’re only mediocre at something and not getting better, spend your time on something you’re the best at. Stick to it, and you can outlast the folks who won’t stick to it. Thus you’ve ridden out the dip of the mediocre and you can be the best.