16 hour light 8 hour dark is what I do. There are many publications on the cyclic requirements of optimal plant growth. The light cycle and temp cycle induce various hormonal cascades in the plants that signal vegetative and reproductive growth. For example, raise an average (always exceptions) tomato at a steady 70F year round and you'll never get fertile fruit. Sure you can leave it on all the time, but a day night cycle would be best for most plants to get a "natural" start to life. There are bunches of studies on circadian rhythm on arabidopsis, everyone's favorite plant model system.
For fish tanks though, unless you have some good co2 injection, I tend to only run 10 hour day cycles.