Hi @ioannis.k
It’s a tricky one 
I read the documentation multiple times and each time came to a different conclusion. 
I found this text in the following documentation that seems to exactly answer your question.
The Absolute Expiration of the rotating refresh token is defined on creation and is not changed, even with an exchange.
Configure Refresh Token Expiration
I interpret this as, Even though you are getting a new refresh token on exchange, the absolute expiry is calculated based on the first refresh token issued in the token family. Meaning after the expiry it would not work and user needs to login again.
So, based on this I would say you get X-1 days of lifetime.
I might be wrong still. 