I imagine Avatar 2 will have a great first weekend, there's clearly huge interest in it. Where it goes from there is a mystery to me, as that three hour plus run time seems like a problem if the story and plot isn't absolutely fabulous and it relies on alien visuals and 3D gimmicks to keep the audience entertained.
It seems to me that after the 1 hour mark, you better darn well have a story and plot that the audience is enthralled by, once the computer wizardry and 3D novelty wears off.
Three hours and ten minutes, with no intermission, is a really long time for 21st century attention spans. I still have a 20th century attention span, and I would probably wander out to the snack bar at the 90 minute mark and may not go back.