I have ones that are 19 years old (Beyerdynamic).
The materials for the main part of headphones would last probably indefinitely if kept in dark, dry place, undisturbed.
On the other hand the foam that is part of the pads lasts months to maybe couple of years depending on use.
If you bought ones with leather it will last for many decades IF you know how to take care of leather.
If you avoid throwing, dropping or otherwise banging headphones on things, don't come outside in them and don't stretch the headband more than is absolutely necessary to put on your head I estimate they can be maintained practically forever (definitely more than user's lifetime).
Maintanence:
- keep them clean with dry cloth,
- use something to clean/protect leather from drying out,
- replace pads maybe every year or two,
- replace pads on headband maybe once every 10 years,
- disassemble completely to remove hair and other detritus, probably every 5 years
- fix the cable that tends to break after prolonged use, I am gentle so maybe once in 7 years.
This is history of maintenance of my headphones which I use every day, entire day.