Thank you for reaching out to Constant Contact API Developer Support, my name is Caleb. My team is here to assist outside software developers with questions about building into Constant Contact's API.
Currently our V3 API does not include the 'expires_in' property in the response as the access tokens have dynamic lifetimes; the expiration date depends on how long it has been since the token was last used.
If the access token is generated but never used, it will have a minimum lifetime of one hour (3,600 seconds). If the access token is used to make API calls, it will expire two hours (7,200 seconds) after the last time it was used. However the maximum lifetime of the access tokens would be 24 hours (86,400 seconds) before it expires and must be refreshed.
To prevent the expiration of your access token we recommend that you set two timers when you generate your access token. One timer counts to two hours and the other timer counts to twenty four hours. Restart the two-hour timer every time you make an API call. If either timer ever reaches its end time, use your refresh token to get a new access token.
Making Access Tokens Last Longer: https://developer.constantcontact.com/api_guide/faqs_manage_applications.html
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