If memory serves, Bjorn sleeps for at least 100 years at a time. He is primarily revived to tell stories of the Heresy; or in the direst case sent to battle. How quickly the faculties fail probably varies from one Dreadnought to another. The type of injury, how powerful their will and spirit are and how old they were when entombed could all play factors. Blood Angel Furiosos are able to advise the chapter command when needed and they are pretty ancient in many cases. However, the Death Company Dreadnoughts do nothing but sleep and fight when their chapter needs them. Further, the Blood Swords' Chapter Master is/was a Dreadnought and he seems/seemed pretty with it.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."