While nothing lasts forever, the reasons for replacement can vary dramatically. Most often, the reason is a color upgrade or surface "glossiness" re-do. Loss of teeth is usually NOT the reason for "failure" 15-20 years later.
Since veneer preparations are very conservative, tooth structure still remains. THIS can decay if not cared for, which could lead to a re-do of the veneer.
At age 25, I would say to expect to have the smile redone 2-3 times. People are living longer, so one should expect to replace or maintain their case.
Most of our cases that are 25 years old or older are being done for no other reason than they want "whiter" teeth, but the teeth are otherwise completely healthy...