In truth it depends on environmental conditions, maintenance and material.These are all made out of metal and the metal will have a long life.The oldest metal swords that have been discovered were buried over 5000 years ago.I am sure the stainless-steel works will last much longer than that given the advances in metallurgy.
Painted aluminum works will last a very long time, although they will require refinishing to keep their shine and luster.

outdoor steel abstract metal sculpture