Exercise 90 minutes a day?

Fat chance.

It figures that the federal government, which isn’t trying to sell books or make friends, would come out with fitness recommendations guaranteed to make many of us snort derisively and inhale a box of Girl Scout cookies.

The U.S. Department of Health and Human Services, which included exercise for the first time in its updated dietary guidelines, fails to realize we’re living in the age of drive-through fitness. If it doesn’t have the word express in front of it, forget it; no workout can be too quick or too painless.

No wonder my bookshelf is crammed with exercise/diet guides that promise to get you in shape in daily workouts of 15, 20 or 21 minutes, or a mere 12 minutes to tighten those trouble spots.

Who wouldn’t want a Hollywood body in only 30 minutes a day? If there’s one constant in all surveys that ask people why they don’t exercise, it’s time.

So there’s instant appeal in a book like Rick Bradley’s “Quick Fit: the Complete 15-Minute No-Sweat Workout. Exercise So Quick and Easy, You’re Bound to Succeed.”

Bradley takes aim at what he calls “The Great Sedentary Majority,” the two-thirds of the public who resent all this yammering about fitness and consider standing up highly overrated.

So here’s his magic formula: 10 minutes of walking, one minute of abs, three minutes of upper-body work and one minute of stretching.

And while the workout’s stripped down, the 900-second routine can be done anywhere without breaking a sweat, which means you can do it in business clothes without having to take a shower after.

Bradley likens exercise to money in the bank – it’s cumulative. Research has shown that breaking exercise into two 15-minute workouts a day or three 10-minute ones does as much for you as one 30-minute session.

It takes about four months to see real results, he says, but you’ll start feeling better almost immediately.

Bradley’s workout seems like a snap compared with the heavy-duty routines outlined in “The Ultimate New York Body Plan” by David Kirsch and “The World’s Fittest You” by Joe Decker.

Kirsch promises a “Total Transformation!” in two weeks, and he’s done it for Heidi Klum, Liv Tyler, model Linda Evangelista and singer Faith Hill.

“The Ultimate New York Body Plan” requires 45 minutes of cardio sculpting – that’s Kirsch’s special blend of 35 intense exercises that either kill you or make you wish you were dead – plus another 45 minutes of additional cardio three times the first week and four times the second.

On the other days, you do 45 minutes of cardio plus a 15-minute workout that targets either your abs/core or your butt/legs.

Kirsch’s fast track to fitness no doubt works, but it’s going to hurt – a lot. At least the book costs only $22; personal consultation with Kirsch for the two-week program at his club runs $7,500. Ouch.

For something edgier, there’s “The Jump Off: 60 Days to a Hip-Hop Hard Body.” Mark Jenkins, a former fat kid turned trainer to rap royalty, bundles the elements of fitness into a driven, down-and-dirty wrapper.

“Other than Mary J. Blige and Diddy, who are both animals,” he says, “almost every celebrity client of mine has thrown up at one time or another.”

Despite that, Jenkins includes well-illustrated exercises, good nutritional advice and lots of motivational blather, which can be helpful.

Kathy Kaehler works with celebs, too, and has named her workouts after them. There’s “Rachel’s Super-Sexy Upper Body Sculpting Workout Starring Jennifer Anniston” and “The Pretty Woman Leg Workout Starring Julia Roberts” to name a few.

“After spending years training with Kathy, I now have the tools to exercise safely and effectively on my own,” Michelle Pfeiffer writes. “She has shown me how to stay in shape without having to spend all of those hours in the gym that are a waste of time and frankly, boring as hell.”