We had to pick up an AP card (already purchased on the app), and this took about 2 minutes at the window a few weeks ago. We had to upgrade tickets to APs in October, and this seemingly very simple transaction (no payment plan or anything at all complicated) took the CM over 45 minutes to complete. No idea why it took that long.
That is to say, some parties will have a quick visit to GS, and others may tie up one GS person for almost an hour without really having a major issue at all. When we went early in the morning in October, there were only 2 windows open (went at 7am, parks opened at 8). When we went in the evening in January (opening night of ROTR, no less), they had 6 windows open. Even still, there were about 4 parties ahead of us in line, and we waited in the line over 30 minutes.
So, give yourself ample time to wait at GS (I would count on 45 minutes to 1 hour to stand in the GS line and complete your transaction). It might be much less than this, but remember that there are some folks who will be there complaining about the fact that they missed out on ROTR the day before when their group didn’t get called etc. Who knows. If you have another person in your party, you could possibly send them to get in line at one of the gates, and then you could join them after you finish at GS (please don’t flame me!). However, if all of you need to upgrade a ticket, you may all have to be present at GS. When we upgraded tickets to Flex pass in October, I let my DD sleep in that morning since we had been at the Oogie boogie bash the night before, and the CM said she could not upgrade anyone’s ticket who was not physically there at the GS booth. So we had a grumpy little camper at GS shortly thereafter!
Good luck and MTFBWY on getting your ROTR BG!!