FYI, we go peak times (week before Christmas, mid-July, Spring Break...) and have always been able to walk into the Frozen Sing-A-Long 10 minutes before. IF you choose to FP it, make sure to arrive at least 20 minutes before the show starts. FP just gets you in first, not a reserved sections, and they let standby guests in at 15 minutes prior to the show start time. We usually walk in to the evening shows less than 5 minutes before start time and get close, but off-center seats on the far side of the theater.
This is NOT true for other shows! We have been turned away from Lion King 35 minutes before start time because the line was past capacity. We waited anyway, just in case, and sure enough the theater filled before our point in line. But, in our experience, Frozen has always had plenty of space.