In one of my large stands a flight(MP0195) which was an Airbus A380 was scheduled to arrive at 12:50 but I saw one of the boarding desk that boarding will opens at 07:41. That’s 5 hours before the plane even lands which is quite weird.
The game world time is accelerated throughout that is the reason. I don’t know the exact time acceleration of the game compared to standard real world minutes/hours maybe one of the developers can confirm for you.
There has been a mention of slowing down the game time slightly as it progresses through BETA testing nearer a full release, but will not become real to life time speed.
Real world flights tend to advertise/start boarding anywhere between 20+ minutes for a small to medium game size aircraft, to anything up to 90-120 minutes for a closed off boarding gate area for larger aircraft. It varies considerably, some airports advertise a boarding gate immediately, other airports passengers have to wait until near the boarding time to find their gate number.