Are you sitting comfortably? You will need to be if you are one of the half million-plus people with a ticket to see the Harry Potter film this weekend.
The trailers alone - before a single frame of the Philosopher's Stone is shown - could take half an hour, drawing out the whole experience to a marathon three hours.
So fierce is competition to hop on the Harry bandwagon that eight studios have tried to lever their trailers into the can. In all, Warner Brothers, who made the film, have agreed to accept 10 trailers, a staggering number - although only three for their own forthcoming films, Scooby-Doo, A Walk To Remember and The Majestic, are being physically attached to the print.
Rival studios have been so keen to break in on the captive Harry Potter audience that the trailer for a new Disney animated blockbuster - Monsters, Inc - is actually an elaborate cartoon homage to Harry.
Fearing that so many trailers will test the patience of parents, Warner Brothers has decreed that the total running time should not exceed three hours, but because of the complex arrangements between distributors and cinemas, they cannot enforce which of the 10 trailers in the can are shown.
Britain's biggest chain, Odeon, said last night that they would show trailers for Monsters Inc, Star Wars: Episode 2 and Lord of The Rings along with the film at their big flagship cinemas, but added that outside the West End the mix would change.
"Some of our bigger multiplexes are showing Harry Potter on five different screens at the same time, and so are trying to vary the trailers depending on what will be showing there later. We haven't set a rule as such, but we think those three will be in the mix almost everywhere. We don't think there should be more than three or so trailers at any one screening," the company said.
Several independent cinemas lucky enough to have acquired a print for the opening weekend have dispensed with the trailers entirely, but their decision has less to do with altruism and sparing the nerves of parents and than with squeezing as many screenings into a day as they can.
A spokesman for the Tricycle cinema in Kilburn, north London, said that with only one screen they had no choice but to drop them if they were to show the film four times a day. "We will be running our normal adverts though," he said. "For us it was a purely financial and practical decision."
With more than 100m JK Rowling books sold already, Odeon's marketing director, Ron Hanlon, is confident that the Philosopher's Stone will outdo Titanic to become the biggest-grossing film of all time.
Odeon alone has dedicated 225 of its 599 screens to the film and trebled its booking staff to cope with the 200,000 people who rang to reserve seats for the opening week. Not all of those clamouring to see the film are children. So great is the demand among grown-ups that the UCI cinema in Swansea is staging adults-only screenings at 11.30pm, a gimmick that other chains are now racing to copy.
Parents already alarmed by the film's two-and-half-hour running time should remember that this does not include adverts, which could add six or seven minutes in which children fuelled with Harry Potter Coca-Cola and Marks and Spencer Hogwarts chocolate will have to be kept amused.
Not everyone is happy with the commercial bonanza that the film has spawned. Tens of thousands of fans have signed a petition to protest about Coca-Cola being allowed to buy the rights to use Harry on its cans in a $150m (£105m) deal from which Ms Rowling is reportedly to get a 10% cut.
