Moose,
You never know for sure.
Maybe only one year if it's not made properly but 30 to 40 years if it is and you take good care of it.
If you can buy a bow and shoot it constantly for even 10 to 20 years, it's not a bad investment.
If you go with a bowyer with a good reputation, you should have no problem finding a long lasting bow.
Sag.