The right time to quit is often something people don’t want to discuss. Most people have been taught to finish what they start not matter what. Back when people actually went to the movies they would sit through the entire movie even if it was bad. After all, they paid good money to be there.
There’s a couple of things wrong with these approaches. First, it assumes that the money that was spent is somehow going to come back to you. It isn’t, so forget about it. Second, it doesn’t take into account all the things you could be doing instead of wasting time on something that isn’t making you happy. If you aren’t enjoying something or making progress then consider quitting. Notice I said consider.
So how do you actually decide? For something like a movie, it’s an easy call. If it’s a bad movie, get up and walk out. You’ll be able to earn the money back, but you can’t get more time. For something larger, like an exercise routine, I’d look at what part of the activity is giving you problems. Ask why you’re stuck. Ask why you can’t make progress. What can you change to make it work for you? Do you need to break something up into small tasks to accomplish them? Maybe you just need a break from it to get some perspective.
If, and only if, you could clearly get more benefit from doing something else than changing what you’re already doing should you quit. Quitting the right things can make space for the things you won’t quit. There is always more than one way to get what you want, you just need to find it.