There is a Studor vent installed under my bathroom sink and lately there is a foul odor coming from it. How long do these valves last?

Hello C Wayne !

Studor vents have spring loaded air admittance valves. If the spring is weak or (mounted) bad, sewer gas may escape into your house.
Check the installation and valve action.
Also (after you first remove the vent) check if the pipe sucks in air or not. If not that may indicate that something else is causing the problem (for instance a poor venting pipe operation)
I add a graphic so you can see how it works !
Feel free to ask for more details!

Success !

:)

If you doubt functionality of Studor vent - replace it.