To create a true wave you have to calculate in fractions of a second, to just alternate the flow, that is up to you. The interval can be what the limits of your device are and most "wave makers" are not making any waves, they just alternate the flow. To re-create the effect of waves on the ocean, you a true wave making device like a Tunze Wave box or pumps that are capable of short, high volume bursts, such as Vortech or Tunze Streams.