Should you quit?Doc, Doghand, BP, Gorilla

You are playing on an honest and deeply dealt DD game. Twenty minutes into playing, the cards were running good, and you won 25 units. Then the cards turn and slowly you start to lose. What do should you do?

Lance Humble once alluded in his book that he quit after winning $400 before losing all back. Is there any truth to quitting theory? It seems to be true on my personal empirical experience.