Last year we had a 7:15 50's prime time ADR after a 6:30 fantasmic.
That Fantasmic was PACKED. Standing room only was full, and they had to turn people away. We planned on sitting relatively close to the exit, but when we entered, we were directed all the way to the back of the stadium, so they could really pack as many in as possible.
We It took a full 15 min to get to Sunset Blvd, so we were about 5 min late for the ADR (which is fine, as PP said, they give you 10 min leeway).
So, even if Fantasmic is packed, and you're sitting as far from the exit as possible... you'll just make it (but I'd recommend trying to sit near the exit, anyway, for peace of mind).
HOWEVER, the were EXTREMELY behind with their seating. We did not get seated until after 8:00 (and it wasn't because we were late, people with 7:30 ADRs who were on line with us to check in got seated after us). We were told the reason for this delay was that "everyone" with an ADR from 7:00 to 7:30 all showed up at the same exact time (after Fantasmic).