When you click on Answering Rules, you will see the option to set the Ring for Seconds, found in the upper left side, below the Answering Rules bar. This is what controls how long a call attempts to ring you.
To set the ring time, click the drop-down to select how many seconds you would like calls to ring before sending the call to voicemail or to the specified destination entered into the When Unanswered field in the active answering rule.
If you do not want a ring time out, meaning you want your phone to keep ringing, select Unlimited at the bottom of the drop-down box.