According to Wikipedia (emphasis mine DVK's, since he edited my question):
The Polyjuice Potion allows the drinker to assume the appearance of someone else for exactly an hour.
Is this supported by canon?
After taking Polyjuice Potion to turn into someone else, how long do you stay in their form before turning back into yourself?
On the order of hours.
When the trio first use the potion in Chamber of Secrets, the book they’re using specifies an hour:
“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)
This is where Wikipedia is getting its statement from – down to the exact wording. But as we learn in Half-Blood Prince, textbooks are not always entirely correct. (Nobody tell Hermione!) The book could be claiming a higher degree of precision than is really justified.
It could also be using a particular variant or method of preparation that pins down the duration – not all potion (recipes) are created equal.
The Pottermore entry suggests a far more variable duration:
The effect of the potion is only temporary, and depending on how well it has been brewed, may last anything from between ten minutes and twelve hours.
This seems more plausible to me – if it really expired on the hour, every hour, it would be much harder to use in secret without provoking suspicion. It’s much more useful (and cheaper) if you can go for hours at a time without needing to top up.
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"
According to the most recent "Word of God", Wikipedia is wrong and the real answer is:
From a Pottermore article written by JKR herself (emphasis mine):
The effect of the potion is only temporary, and depending on how well it has been brewed, may last anything from between ten minutes and twelve hours. You can change age, sex and race by taking the Polyjuice Potion, but not species.
There's a strong canon indication it may be able to last far longer than an hour, no matter what Hermione's book said.
Harry takes Polyjuice to be able to attend Bill and Fleur's wedding in DH.
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.
And the wedding (started at 3pm) lasted WELL into the evening, meaning far more than 1 hour passed, and Harry's Polyjuice still worked - we didn't see him taking swigs of it out of a flask ala fake-Moody :)
... As the evening drew in, and moths began to swoop under the cnopy, now lit with floating golden lanterns, the revelry became more and more uncontained. Fred and George had long since disappeared into the darkness with a pair of Fleur’s cousins; Charlie, Hagrid, and a squat wizard in a purple porkpie hat were singing “Odn the Hero” in the corner.
In the Chamber of Secrets the gang first takes the PolyJuice potion, based off of what we can assume is the most common form or variant, if multiple exist.
“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.”
In Goblet of Fire, Barty Crouch Jr. uses the same form of Polyjuice as Harry and Co. and was required to drink it every hour to maintain his appearance.
"Polyjuice Potion, Harry," said Dumbledore. "You see the simplicity of it, and the brilliance. For Moody never does drink except from his hip flask, he's well known for it. The imposter needed, of course, to keep the real Moody close by, so that he could continue making the potion. You see his hair ..." Dumbledore looked down on the Moody in the trunk. "The imposter has been cutting it off all year, see where it is uneven?
But I think, in the excitement of tonight, our fake Moody might have forgotten to take it as frequendy as he should have done ... on the hour . . . every hour. . . . We shall see."
In addition to the previous answers, when Harry, Ron, and Hermione infiltrated the Ministry with Polyjuice Potion in Deathly Hallows, the potion didn't wear off until they got to the Quidditch World Cup location, which would have been well more than an hour after they had taken it.
The Polyjuice Potion was wearing off now: Ron was halfway between Cattermole and himself in appearance, his hair turning redder and redder as his face drained of the little color it had left.
