Officially, self-disembarkation happens first. You haven't specified but, assuming Iona since you are going to the Fjords, self-disembarkation has been between 7:30 and 8:15 on my last two Iona cruises. However, it has generally been possible to get off before this.
I've found self-disembarkation busier on Iona than any of the other ships due to the often younger demographic, although I do only travel during the holidays.
After self-disembarkation time, everyone is supposed to go to their allocated meeting points and be disembarked in groups. I have never tried to self-disembark after the allocated time to advice if this is possible but you no longer have to show your disembarkation papers as you did years ago when leaving the ship.