@crimsonkid The session on System B is in no way tied to the token lifetimes in System A. The token can be revoked in System A any time before the session ends in System B, and a session can expire in System B any time before the token expires or is revoked in System A.
OAuth tokens (and tokens in token-based authentication) are stateless. Therefore you can’t (and shouldn’t) pin their lifetime to a stateful authentication mechanism like a session in a completely separate application.