The roots only need to be watered every 15 minutes or so. The plants don't need to sit in the
nutrient solution longer than it takes to fill the garden. The overflow is provided for people with timers that run longer than it takes to just fill the garden. If it takes 3 minutes to fill the garden to the correct height then set your timer to come on for three minutes every 15 minutes. That is if you can set your timer to do that.