There are numerous types, profiles, manufacturers, and other non-product related variables that will determine how long of a warranty a steel based roofing product will come with.  Typically, a prefinished (painted) finish will come with a 20 year paint finish warranty as well as a 20 year corrosion warranty. However, if installed within a certain proximity of salt water this warranty will likely not be available.  Some of the Stone Coated steel products such as Gerard or Decra-Bond offer a 50 year warranty.  These products do not carry many of the limitations that a pre-finished steel product will contain.