Hm. You might be done, if you want to be, esp given that it's a buffet. We had an 8:15 res last week and I don't think we were there for an hour. More like 45 min or thereabouts. But we were seated in the area where the characters come first, and they alerted us to this (so that we made sure to be around when they came by rather than stuck up at the buffet). In our case the park was open for EMH, so we got our FPs before we went in and walked right into the line when we were done. That should give a rough estimation of how long it took, since the FP is for minimum...45 min later? I dunno the minimum, but I think the earliest validity is something like 45 min from the time you get it, even if the standby wait is only 10 min. I want to say the FP said 8:50-9:50, and we were done around 9:00.
Just play it by ear. May depend upon where you are seated and when the characters get to your table. Also, arrive early, they may seat you earlier than your res time depending upon how the tables are reserved that morning. We were at the park before 8, went in at 8:00 opening, and went straight to Tusker. The restaurant host didn't tell us to hang back until 8:15, anything like that. In fact after two ADR character breakfast res my impression at the moment is that the res time is more of a suggestion or approximation, and the order in which you arrive actually plays a fair part in when you are seated. (Arriving an hour early, probably not. But 15-20 min early? Yeah, if there's a clear table, you're probably good. So get to AK early, like about 7:45, so you can get down to Tusker around 8 and see what happens.) The other one, at Akershus, was well past the stated time before we were seated. At Tusker we had already taken our pic with Donald and were sitting down by the res time (or pretty close to it).
If it looks like you may be there a while longer, then someone can go out at 9 and get FPs. If you might finish close to 9, you might want to go out and just get in the standby line. It's literally right outside. Even if you get out right at 9, you'll still beat the people who are walking in from the entrance as that takes a few minutes even running.
