I think FOP opens every day with a "posted" wait of 90 minutes or so. The strategy about arriving ~1 hour before park open is the best one. Last year, we visited on a morning with extra magic hours starting at 8AM and got to the ticket booths at 6:55AM. My wife told me 'I think we overshot this a little bit' - but 10 minutes later, they opened the gates and started letting the crowd in to the park. The folks showing up at 8AM then showed up to find the 120 minute line previously mentioned, while we were on and off the ride before 8AM when the park was expected to open.
If you rope drop and are there early, don't be alarmed at the crowds and line outside of the show building in Pandora. Nearly everyone stops in the queue to take a photo just outside of the show building and it slows everyone behind them down. In the morning, once you reach the building, you are usually going to be able to ride within 10-15 minutes.