Your choice of plants and the water holding capacity of your substrate are the determining factors for how often you should flood and drain your rig. For an absorbent growing medium with low needs plants three to five cycles a day will likely do the job. Thirstier ones or those in inorganic material may need up to eight cycles a day. The temperature inside your grow room also plays a part. Warm weather makes for thirstier plants and will also dry your medium out more quickly. Keep an eye on your plants and they'll let you know if you need to up the frequency. Watch for classic signs of dehydration like limp leaves or drooping stalks.