I believe a well made, well cared for and carefully used osage bow will easily last a lifetime. I have some that are >10 years old that have shot thousands of arrows, and besides being darker brown, they look and perform the same as the day they were made.
I also have a 50's or 60's vintage Bear static recurve that has osage as the core. It's FG limbs mind you, but it is in mint condition and still gets shot regularily.