Have a look at the following:
FL-107 Gyro Erection Limitations Course
1journey.net/1candle/av/FL-107/FL-107.htm
Item 4 in: False Inflight Topples.
Wish i had read this article when i was working on aircraft.
Sperry Vertcal Gyro's VG-14 showed false horizon when on constant bank angle 20 minutes or so. It was recommended to do figure 8 turns if in long holding pattern.