Markets can remain irrational a lot longer than you and I can remain solvent.

John Maynard Keynes (1883-1946) English economist

(Attributed)

Generally credited to Keynes, but the earliest reference found is by financial analyst A. Gary Shilling, "Scoreboard," Forbes (15 Feb 1993). More discussion here.

Sometimes given as "Markets can remain irrational longer than you can remain solvent" or "Markets can stay irrational longer than you can stay solvent."