When is the right time to quit something? A startup, a marriage, a book, etc. Is there a right time? If not, when should you quit? Or should you not?
The people who make it say that persistence is key.
But what if you're climbing the wrong mountain? How/when do you know that what you need most is change?
For me personally, I've changed things when my heart just isn't in it for the long haul anymore. When I don't love it anymore. (And usually, when I realize I don't love it anymore, I often realize -- sometimes not until much later -- that I never did.)
It's been clear to me when I do love something, or someone. So that's what I focus on.
When you really want to quit, set a deterministic goal to hit in say six months. Give it your all with your last reserves. If you hit that goal, your energy will come back. If you don't hit that goal after six months, then you know you gave it your best. Pursue the contingency plan of posting your CV on Whitetruffle or LinkedIn as a startup founder/early employee and score a six figure offer within days.
Take the offer, heal up and lick your wounds for a few years, figure out what you did wrong...and remind yourself and everyone else why you truly are God's gift to hacking by utterly killing it at your new company.
The principle, "Exits are harder than entries," comes up in a lot of different places.
Hindsight bias distorts the picture. What could be more obvious than that, say, the Twitter guys were right to keep making Twitter even when nobody else cared. But whenever we think that way, we're retroactively assuming the power to predict the future. Then we find ourselves in some real situation that matters and that superpower doesn't work and we're totally stumped. (Edit: actually almost all our vocabulary around "persistence" assumes hindsight. When something turns out to have worked, you displayed plucky perseverance. When it fails, you were a stubborn fool. In real time, these qualities are not so easy to tell apart! Maybe they're the same thing sometimes.)
So what do you do? It's a hard, deep question. I agree with stevenj. Consult the heart.
Clearly there are tons of people excited about the idea of Adioso. The problem is that the technology doesn't deliver. No matter how ridiculous of a schedule I'm willing to put up with, I still can't get from NYC to Cambodia and back for $300 or whatever. And it's not even clear that that will ever be possible no matter how much time they put into developing it.
Now if they were able to figure out a way to charter planes that would otherwise be flying empty and get them filled to capacity so that I actually could go to some random country in SouthEast asia for a few hundred bucks then that would be awesome. But that simply hasn't happened yet.
Seth Godin's The Dip: A Little Book That Teaches You When to Quit (and When to Stick) [1] is an oft-recommended resource on the topic, though I didn't find it particularly helpful the last time I was in a similar situation.
On a startup, the concept of engines of growth[1] can be helpful determining if you are reaching product market fit (ie: you should stick with it) or not (you should change something[2] / change to something completely different).
The parallel to hill climbing algorithms: it serves as an heuristic to determine if you growth rate will be enough to reach product market fit.
The people who make it say that persistence is key.
But what if you're climbing the wrong mountain? How/when do you know that what you need most is change?
For me personally, I've changed things when my heart just isn't in it for the long haul anymore. When I don't love it anymore. (And usually, when I realize I don't love it anymore, I often realize -- sometimes not until much later -- that I never did.)
It's been clear to me when I do love something, or someone. So that's what I focus on.