leafy via www.flickr.com creative commons
It's not helpful, since customers have no idea when your countdown began, and therefore don't know whether they will have to wait a matter of seconds or most of the posted time - or longer.
Indeed, let's be honest, in most cases those signs underestimate how long a store will be closed. Case in point was the Williamsburg record store I walked past several times last Friday night that had a "Back In 5 Minutes" sign posted for at least 20 minutes.
This happens because humans have a tendency to underestimate time and because workers may take advantage of the ambiguity to gain a longer break.
Indeed, I've used that trick myself, when I worked at an art gallery during college and realized that I could extend my 15-minute break by at least 5 minutes putting an ambiguous "Back In 15 Minutes" sign on the door.
Now I realize how rude it is to leave customers in limbo.
As for people who post that sign without manipulative intent, it would be vastly more useful if you could post the time when you expect to return instead.