When exactly is it okay to quit? I think it depends on what it is you’re quitting. If it’s a bad habit, like smoking or even picking your nose, then any time is a good time. Certainly if your bad habit is getting on the nerves of your nearest and dearest and you know it is, then you shouldn’t hesitate to start packing it in. You not only want to please your family, you want to keep your friends too.
When it comes to your job, it’s best to quit when you feel the time is right and you’re ready for a new challenge. It’s always best to make sure that you have another job lined up as well before you do it – unless you’re retiring when of course you can do as you please within reason. And of course, you also want to leave on good terms – even if the reason for leaving is because you have got increasingly annoyed by the job and/or some of the people you work with.
For everything else, I’d like to say that no matter what it is you’re trying to do, be it writing a novel, becoming a successful pop star or even mastering the art of blowing a bubble gum bubble, you should never give up on them as you’re bound to regret it. But if you are absolutely sure that you will never, ever succeed in what you want to do, then go ahead – but know that there’s no harm in trying again sometime in the future.