Det kan vara relaterat till dosstorleken. I Secret of Secrets hävdar Hermione att effekterna kommer att ligga i exakt en timme:
"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"
Och effekterna verkar vara i en timme; De spenderar några minuter i tvättrummet, sedan om femton jakt på Slytherins gemensamma rum innan Malfoy räddar dem och sedan en ganska lång konversation. Även om vi inte får veta exakt hur länge de spenderar i förklädnad, verkar en timme trolig.
Å andra sidan, som DVK påpekar verkar Harry förklädd av en enstaka dos av Polyjuice för en hel kväll. Detta kallas emellertid uttryckligen som en "stor dos" (betonar min):
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"