Unless there is a need to run the filter , ie cleaning up after an algea bloom , 15 hours is not beneficial.

What happens normally is the stuff in the pool either sinks or is suspended. The stuff that sank isnt going anywhere but the bottom. The suspended material will be picked up by the filter . Eventually , when all of the suspended stuff is cleaned out , clean water is being filtered all over again.. not beneficial

Actually , cycling the hours the filter runs is most beneficial , say something like 6 hours on 6 hours off 6 hours on etc etc