Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
Access tokens have a lifespan of 24 hours and would need to be refreshed daily if your application is in consistent use. Below I’m linking our OAuth2 Overview documentation which includes detailed information about our access and refresh tokens.
Please have a look and let us know if you have any other questions!