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.
Busy days:
If you go to the first show of the evening (or "only" show that evening), if you arrive about 60-70 minutes before showtime, 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.
Off-Season (not busy days):
You should be OK to enter by 15 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.
Thats'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!)