Auth Bearer expiry/validity

What is the duration of the authentication bearer validity? I see that the validity is over 50 years "expires_in": 1576799999 as stated in this other topic: OAuth access token expiry and refresh token API but the code I had expired couple of times now - one expired in 7 days, the other in 24 days so, I couldn’t tell the standard expiry.

This was the error message I received when the token expired:

{"error":true,"type":"AccessDenied","message":"This request is not authorized by the specified access token"}

Still waiting on any pointers on how to resolve the bearer expiry issue. Greatly appreciated.