This is a setting found in your dashboard within the editor for a virtual number, menu option, or extension, under "Forwarding Settings". The only time you will see this setting is if the virtual number, menu option, or extension is set to forward to your phones, and the "Ring All" forwarding strategy is being used.
The system rings your phones in 30-second segments called "cycles". While your phones are ringing, the How often should we try to forward calls to your phones? setting determines the amount of time the system will wait between ring cycles.
If the maximum hold time = 2 minutes, and the How often should we try to forward calls to your phones? setting = 15 seconds, then the ring pattern will be as follows:
Ring 30 seconds—>wait 15 seconds—>ring 30 seconds—>wait 15 seconds—>ring 30 seconds
*It will repeat this pattern until the call is answered, or until the maximum hold time is reached.