How long does it take to get out of shape – de-condition?

For me, it seems like about three days. Here's a more scientific look. It's a short article, only 11 paragraphs if you hit the link.

. “If you stop resistance training, you will lose strength at about half the rate it was gained,” explains Pete McCall, M.S., C.S.C.S. and exercise physiologist for the American Council on Exercise. “So if you had increased your leg strength by 50 percent after following a 10-week program, then stopped working your legs completely, you’d lose half of that strength in 10 weeks, and all of it 20 weeks later.”

“Studies have shown that cardio fitness tends to go away quicker than fitness built by strength training,” says McCall. In other words, that Spinning class is going to feel tougher than lifting weights.

And lastly, other factors come into play when your muscle definition dwindles. Things like genetics, age—we lose three to five percent of muscle mass per decade after we turn 30, says McCall—stress levels, sleep, and metabolism all have a role in how quickly you’ll lose that strength.