Been standin' behind a lot of jets lately, eh Unanswerd?
I'd say that a safe distance to stand behind a jet would be however far it takes to keep from getting blown over. As a general rule, most of us avoid standing directly behind aircraft with engines running as it is a rather hot, windy, noisy, smelly, and altogether uncomfortable place to be. This practice of avoiding dangerous and uncomfortable areas is often referred to as "common sense." You may have heard of this before. Unfortunately, common sense is neglected in certain circles. If I were king of aviation, every student would have to demonstrate common sense as a part of their medical examination, but it obviously doesn't always work that way.
I trust that this has been an informative post, although I strongly suspect you could have come up with this information on your own.
-Goose