I have a frame hitch on my 1976 Ford F150 that was installed in 1976, used regularly, and is still in fine condition. Another on a 1972 brand X (before I parted ways with GM over their worthless warranties) that is still just fine, last time it was used a good while back. As previous posters have indicated, steel is steel, and as long as it is not damaged, either by distortion or rust, it will remain steel, with the same strength it had when new. The ball may wear out if not greased, but the hitch will be fine. Even balls are good for a long time - I have one that was purchased by my grandfather somewhere in the 1940's that I still use with no problem.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.