This is a quote I use very often! As with many other great quotes, I think it originates in English from Trevanian's Shibumi (when Otake-san, the Go master is giving his final teaching to Nicholai):
Do not fall into the error of the artisan who boasts of twenty years experience in his craft while in fact he has had only one year of experience — twenty times.
Most probably Trevanian has drawn upon some other source, tough I've never been able to track that.
If you haven't read this fantastic book I strongly recommend it.
It's an ok potboiler that relies ever-more improbable plot devices.
It's also impossibly incorrect politically -- so transparently so that I think it's essentially harmless. But if you don't understand why some people would feel that why you really should watch your step.
David Courtwright used a slightly different version of it to describe the Vietnam war: "we were not in Vietnam for 10 years, but for one year 10 times."
I believe the origin is in medicine. My wife introduced the idea too me 10 years ago. In medical practice, watch out for the doctor who's been practicing the same year of medicine over and over and over.
I've heard it from one of the karate instructors I've corresponded with (although in that context it was less of a dismissive phrase and more a cautionary one for our own training). Regardless of the origin, it's a good one.