Last time i went we started queuing about 2 hours before, it was before they had opened the gate. We got the best seats but there was about 1000 people in that queue then so the best seats were gone straight away and it appeared to be full about and hour before showtime.
My WTP (Wait Tolerance Point) is about 45 minutes. Unless Joe buys me cotton candy and a pretzel - then my WTP can go up to 60 minutes. And I get fidgety at that...I can only people watch and do the wave back and forth so many times before getting bored 
I tried to pass the time buy reviewing the days photos on my camera, until OH pointed out i was draining the batterys and wouldn't be able to take any Fantasmic photos.
For my trip next month Disney has changed the hours at DHS and there will be 2 showings of Fantasmic. This helps since I'm taking 2 newbies and they have to see it. I'm just going to park hop to DHS 1 night with the sole purpose of seeing Fantasmic. Should make it easier for us. I'll probably get in line about an hour before and let them wander around while I wait in the line.