It may be related to dosage size. In Chamber of Secrets, Hermione claims the effects will last for exactly one hour:
"I'm sure I've done everything right," said Hermione, nervously rereading the splotched page of Moste Potente Potions. "It looks like the book says it should... once we've drunk it, we'll have exactly an hour before we change back into ourselves."
Chamber of Secrets Chapter 12: "The Polyjuice Potion"
And the effects do appear to last for an hour; they spend a few minutes dawdling in the washroom, then about fifteen hunting for the Slytherin common room before Malfoy rescues them, and then a fairly long conversation. Although we're not told exactly how long they spend in disguise, an hour seems plausible.
On the other hand, as DVK points out, Harry seems to be disguised by a single dose of Polyjuice for an entire evening. However, this is explicitly called out as being a "large dose" (emphasis mine):
Three o'clock on the following afternoon found Harry, Ron, Fred and George standing outside the great white marquee in the orchard, awaiting the arrival of the wedding guests. Harry had taken a large dose of Polyjuice Potion and was now the double of a redheaded Muggle boy from the local village, Ottery St. Catchpole, from whom Fred had stolen hairs using a Summoning Charm. The plan was to introduce Harry as "Cousin Barny" and trust to the great number of Weasley relatives to camouflage him.
Deathly Hallows Chapter 8: "The Wedding"