ensure that both new and old access_token values are available within five minutes, so that third-party services are smoothly transitioned.

WeChat public doc https://developers.weixin.qq.com/doc/offiaccount/en/Basic_Information/Get_access_token.html

 

 In this case, the backend of the Official Accounts Platform will ensure that both new and old access_token values are available within five minutes, so that third-party services are smoothly transitioned.