The wide application of the Internet of Things drives research to concentrate on the security aspect of low-power wide-area networks (LPWAN). In the long-range protocol, LoRaWAN 1.1, the rejoin method was introduced to enhance the security of communication by refreshing the session keys that are used to ensure confidentiality and integrity. However, this approach has a drawback of occupying the communication channels, affecting the performance of the entire system. In this paper, an evaluation method, namely the Airtime Occupancy Rate (AOR), is proposed to quantify the communication overhead, or the occupancy of channel resources, of rejoin. The classifications of the communication overhead are also introduced to evaluate periodic rejoin schemes.