You cant know for sure until park hours are posted on the disney website.. they change year to year. BUT fantasmic usually runs during the last hour the park is open.. but plan to que up far in advance. WE lined up 90 minutes early to get GREAT seats.. but no less than 45 minutes early to just get in!
I just looked at last year and it was at 9:00p and 10:30p the end of May (Memorial Day weekend) and 9:00p in the beginning of June with some nights having a second show at 10:30p.
If you prefer not to wait that long check out the dining package. We have done that 3 times now and love it. We are going to eat anyway, and Mama Melrose is excellent, so we do the fantasmic package and only arrive 30 mins or so early. In my opinion there are no bad seats for the show, the package seats are on the left side of the theatre and make for a good exit at the end of the show.