General rule is to clean out and start fresh every 7-10 days.
Usually what happens is once you start fresh with a new batch of mixed nutrients, the water that evaporates or is taken up by the plant will get replaced with pure pH balanced water.
I know a few people who every month break down parts and peroxide bath them in between to keep everything nice and clean. This isn’t needed but keeps everything nice and clean throughout the grows. However, once you are finished with the grow I would deep clean the whole system before starting again.
Hope this helps.