To be clear there are a wide variety of steel targets. Some are smaller and made for .22lr. Some are made for larger calibers and include such things as shooting trees. There are also heavier targets used for rifle shooting (at distance). Finally there are a pretty formidable style used in competitions (they are typically much more rugged and heavier).
Is this specifically what you are talking about?
http://www.shootsteel.com/3-8-ar500-...target-7x11-8/
It says: "Handguns should be used no closer than 10 yards and rifles 100 yards." That seems pretty appropriate. I'd lean a little closer to 15 yards though. I hate being hit by bits of metal.