Assuming you are talking about dining at MGM, each restaurant is different. Most make the switch over around 3:30 or 4, but your best bet is to look on the disney site which lists the hours of each meal.
FANTASMIC! Eatery times for 1st Show (no listing for a 2nd show, as such is seasonal):
. . . Mama Melrose, Fantasmic! Lunch, 11:30am-4:15pm
. . . Mama Melrose, Fantasmic! Dinner, 4:45pm-5:30pm
. . . Hollywood Brown Derby Fantasmic! Lunch, Noon-3:35pm
. . . Hollywood Brown Derby Fantasmic! Dinner, 3:30pm-6:00pm
. . . Hollywood and Vine Fantasmic! Lunch, 11:00am-2:55pm
. . . Hollywood and Vine Fantasmic! Dinner, 4:15pm-6:30pm
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