Dawn to dusk photocells seem to be on VERY much longer these days. They are about 15 years old. Do these kind of devices have a practical lifetime?

They do eventually loose their sensitivity. I have never had one last 15 years so maybe you are the lucky one. They are easily replaced, what is it connected to ?

If the trees have grown in 15 years, it will be darker and photocell will see dark sooner.
Maybe relocate photocell?