Hi there, I'd say that arriving at 8PM is not a problem in terms of being able to catch a shuttle van to Antigua. Even at a later hour you can always share a taxi or private van which normally comes up to about $10/15 which is the average shuttle fare. Clearing inmigration and customs in guatemala is real fast. Now on your departure day, like I told you on the other post, you can always catch an early shuttle van to the airport. Antigua is about a 45 minute ride on a well travelled four lane highway. All the flights to Flores/Tikal leave at 6AM. So, there's always pax going to the airport at 4AM and catching your 6:25 flight should not be a problem. Now arriving try to have an advance hotel reservation on a idea of where you want to stay in Antigua. There are plenty of hotels in Antigua. Thus, arriving without reservations should not be a real problem. The other pax on the shuttle can always tell you what's a good place to stay anyway. Good luck.

I'd want to be at the airport a bit earlier, just to be sure. 4:00 a.m. pick up in Antigua should be OK, IF you're the last pick up. But if you're the first, you may not be on the road 'til 4:30.

On the other hand, you might just need to consider the possibility of spending your last night or so in Guatemala City. Antigua has a way of really sucking for some people, especially for those who like street signs that are useful. I try to stay away from Antigua myself.