Q:

What unit is the Marketing Cloud access_token expires_in value using? 

When I send a request to this endpoint, https://my-subdomain-info.auth.marketingcloudapis.com/v2/token, 
This information comes back:
"access_token": (I removed the token from this post.)  
"token_type": "Bearer", 
"expires_in": 1079 

What does the 1079 refer to?  1079 what?  

A:

It is seconds.  The lifetime of the token is 20 minutes (20*60=1200), as per this documentation: https://developer.salesforce.com/docs/atlas.en-us.mc-app-development.meta/mc-app-development/access-token-s2s.htm
Normally I store the expiry timestamp in the system integrating with SFMC, which allows me to generate a new token well before the expiry.