It always depend on the time required to become dirty. One thing for sure is if you wait too long and you have debris and algae stuck in the weir, the overflow will become uneven and it can cause your clarifier to get more water coming out of it at different place ( water speed increase) and maybe more particules in the effluent.
Daily with water spraying. Routine plant wash downs. Hopefully your plant is equipped with decently high pressure supplied to plant hose bibs. Some plants are small and only requires a quick brushing once a week. Depends on your plant.