I agree. I remember reading Seth Godin's book "The Dip" and the same conundrum is presented: when do you push through the dip and when do you quit?
He says you should quit when you're in a cul-de-sac or heading towards a dead end, both ultimately ending unfavourably. A dip, on the other hand, has a low point that feels like a dead end but inevitably gets better the longer you persist.
A good, short read that gives some suggestions to deciphering which is which if you're interested.
"Your twenties are not a time to make money, they are a time to build your foundation; your thirties are when you make money."
The exact ages are peripheral, but the idea is sound: incur personal strength and experience and external success will (be more likely to) follow.
Mark Suster's post about a time to learn and a time to earn hints at this same idea: http://www.bothsidesofthetable.com/2009/11/04/is-it-time-for...