People like to say that around the six- to eight-week mark, your body adapts to whatever workout you’ve been doing and stops getting the same results. At that point, the rumor goes, you have to up your game by adding more time, weight, new strength moves—anything that changes things up. But what’s so special about that specific length of time? And is it really true?

Turns out, not so much. “There’s nothing magical about that number. It’s a myth,” says Richard Weil, M.Ed., an exercise physiologist and director of the weight loss program at Mount Sinai St. Luke’s Hospital in New York City. “The fact is that research-based guidelines on how often you need to vary your workout to avoid a plateau just don’t exist.”

So where did this idea come from in the first place? The basic principle was actually developed by a Hungarian endocrinologist named Hans Selye, way back in the 1930s (yep, that’s right—looong before CrossFit and Megaformers). “He theorized that muscles needed continual stress to change. Otherwise, they would adapt,” explains Pete McCall, a San Diego-based personal trainer and exercise physiologist. “It’s called general adaptation syndrome—and it’s what the basic science of periodization is based on.”

Furthermore, neural adaptations are most prominent in the early phase of training when our body is learning how to do the moves effectively and efficiently; those become less prevalent around eight weeks of training. Structural adaptations, however, continue to occur past the eight-week mark, so switching programs at that time limits the very increases in muscle strength you set out for. The human body is efficient, so continuing to challenge it by making small changes (such as the number of repetitions or duration of time) to the same routine is ideal for using both neural and structural adaptations.

To be clear, McCall says it’s true that as you get stronger or more aerobically fit, you’ll eventually hit a point of diminishing returns. “Your body becomes more efficient at doing the same exercise and your muscles go on autopilot because there’s no new stimulation,” adds McCall. When that happens, however, can vary wildly from person to person.

“There are so many variables: how often you work out, what shape you’re in, how much weight you’re lifting—even genetics plays a role,” Weil says.

You’ll know you’re due for a workout goose, he says, when your current routine starts to feel easier. Say you’ve been doing 12 to 15 reps of an exercise for a while, and you get to a point where you can finish the last rep, no sweat. Bingo—you’re due. (The goal is to barely be able to eke out that last rep.)

Same goes for cardio: When your usual elliptical routine doesn’t get your heart rate going in the same way, it’s time for a change. And it’s worth noting, McCall says, that even doing something less intense, like switching from boot camp to Pilates, can do the trick, simply because it’s something new and different—and therefore more challenging.

But keep in mind that fitness doesn’t increase indefinitely. After exercising regularly for a certain amount of time—usually between 12 to 16 weeks, for most people—you physically won’t be able to get much fitter. “Everyone has a biomechanical max that they can’t go past,” Weil says. “For instance, you have a certain number of muscle fibers in your biceps, and once you accumulate as many as you possibly can, you can’t gain any more strength.”

Think of it less as a plateau than a pinnacle. At that point, it’s all about maintenance, and whether or not you change your workout altogether doesn’t really matter—unless boredom strikes. “Look at elite marathoners. Do you think they’re doing Zumba? No! They’re running all the time. They never change their workout,” Weil says. So if whatever you’re doing is something you like—and are more likely to actually stick to—well, then that’s your real ticket to long-term results.

Shaun Dreisbach is a Burlington, Vermont-based writer and editor who specializes in health, fitness, and nutrition.