code: func (c *cubicSender) TimeUntilSend(bytesInFlight protocol.ByteCount) time.Duration {
  	if c.InRecovery() {
  		// PRR is used when in recovery.
  		if c.prr.CanSend(c.GetCongestionWindow(), bytesInFlight, c.GetSlowStartThreshold()) {
  			return 0
  		}
  	}
  	return c.rttStats.SmoothedRTT() * time.Duration(protocol.DefaultTCPMSS) / time.Duration(2*c.GetCongestionWindow())
  }
docs: // TimeUntilSend returns when the next packet should be sent.