SO LET’S GET DOWN to the most urgent lesson of the summer movie season: Suddenly, there’s no such thing as shelf life.

Remember the days when “ET” rode the top of the charts for 10 weeks? Or “Ghostbusters” for eight? Well, welcome to the era of the one-weekend wonder. A movie better open big, because it will be gone in 60 seconds.

This summer, only two films — “The Mummy Returns” and “Pearl Harbor” — managed to levitate a second week atop the charts. Virtually every other film that opened at No. 1 crashed 50%-60% in the second week and quickly dropped from sight.

So what’s happened to moviegoers’ attention spans? Has some sort of self-destruct mechanism been built into the new releases?

Talk to the town’s decision-makers and you quickly learn that the one-weekend wonder has been accepted as a fact of life. No one is saying, “This trend sucks. I’m going to come up with a different sort of movie that will reverse this pattern.”

Rather, dealmakers are studying accommodations that will impact filmmakers, distributors and especially hapless exhibitors.

Accommodation No. 1: What sort of films succeed in this new environment? A glance at the success stories of the past two summers suggests that sequels and high-concept movies best stoke the one-weekend fever. A “Jurassic Park III” or a gimmick picture like “Planet of the Apes” is ideal grist. Star vehicles don’t seem to register because special effects are the stars — Ben Affleck, Sam Neill and Mark Wahlberg were the “superstars” of summer.

WHILE SOME QUESTIONED the viability of Sony’s “Men in Black II” at a cost north of $140 million, a marketing-driven project like this seems thoroughly justifiable in this climate. The catch: If the excitement is more in the campaign than in the movie, the public’s attention will shift to the next entry with lightning speed.

The upshot of this strategy is that the so-called “serious” films — i.e. those aimed at adults — are ghettoized at the end of the year. This December, more than 13 high-profile movies with estimable casts and budgets will open in the last two weeks of December. It’s as though movie theaters were becoming like schools, with December emerging as “parents’ day.”

Hence, even in December, bigger marketing and special-effects budgets are the order of the day.

“I’ve had my problems getting approvals to spend that last $5 (million) to $10 million on effects,” says the production chief of one major. “Now resistance has faded. The lesson is that you have to spend the big bucks or get buried.”

The impact of the one-weekend wonder has been especially daunting at the multiplex. A movie like “Planet of the Apes” may play on six different screens, starting every hour — a sort of in-theater video-on-demand. Filmgoers have to look hard to find a stray screen playing a “serious” movie. The reason is that theater owners are desperately seeking to milk every penny from every wannabe blockbuster, mindful that its audience will vaporize after the first weekend.

All this has a nightmare impact on exhibitor profits, whose piece of the pie may start at, say, 30% of the box office take and rise with every passing week.

This formula worked in the old days; by the fourth week the exhibitor had a significant rooting interest in his new hit. Today, of course, there is no fourth week. The rewards are as slim for the exhibitor as for the filmgoer.

As I said before, welcome to the era of the one-weekend wonder.