All Fantasmic! shows are listed on the WDW Calendar.
http://disneyworld.disney.go.com/wdw/calendar/parkhours/index?id=MKCalendarParkHoursMonthlyViewPage
Pick your Park. Pick your Month. Click on your Day.
Disney publishes dates and times approximately 6 months in advance.
Make sure to check back often, as times and events are subject to change as the actual dates get nearer.
There is no "LINE" for Fantasmic! (at least for the first show).
Although if you arrive a bit later, you will be "shuffling in" amid a large slolw-moving GROUJP, but no "line").
The theater is "opened" about 60-90 before show-time and guests can just walk into the theater and pick a seat.
No one can predict EXACTLY what will happen on any given day.
It has totally to do with how much time you wish to invest, and the general crowd levels.
VERY Busy days:
If you go to the first show of the evening (or "only" show that evening), if you arrive about 75-90 minutes before show-time, you can sit just about anywhere.
45 minutes before: Good seats
20-30 Minutes: will vary between seats at edges and rear of theater to SRO or even "closed" to further guests.
Busy days:
If you go to the first show of the evening (or "only" show that evening), if you arrive about 50-60 minutes before show-time, you can sit just about anywhere.
40 minutes before: Good seats
25-35 Minutes: will vary between seats at edges and rear of theater to SRO or even "closed" to further guests. (The crowd of "last-minute" arrivals can be slow to get seated before show-time.)
Off-Season (not busy days):
You should be OK to enter by 10-20 minutes before, but you'll likely be seated to the outer edges, and/or way at the back.
If you do the dinner pkg. you can enter 30 minutes before (or a bit later) and still be guaranteed pretty good seats, regardless of crowd-levels.
If there are TWO Fantasmic! shows in one evening, the second one will have lower crowd level than the first one. You can arrive a bit later and still find a seat.
Be aware that the second show waiting area will LOOK like the show is too crowded.
That's because they HOLD the 2nd crowd outside the theater while the CMs CLEAN it after the first crowd has exited.
Then, they will let those waiting together in that big group IN... all at once, and there will be PLENTY of room.
(The theater holds 10,000 guests!)
We always plan to eat our evening "meal" IN the Fantasmic! theater.
They have "baseball stadium"-type concession stands IN the theater.
We have a hot dog and a giant pretzel and some soft drinks in our seats as we wait. (There's even beer, sold strictly one-at-a-time, one-to-an-adult).
Just outside the theater entrance is the large "food court".
You are allowed to carry in food from there as well, but remember that its a LONG WAY up the sidewalk from there to the theater itself.