Sci Fi took forever for us when we were there in February. I don't know what the deal was as we just ordered sandwiches (it was lunch time) but it took us a solid 2 hours from the time we walked in until the time we walked out.
Depends on how late they are running, are you seated on time or closer to 6:30, etc. An hour at least is a safe bet for all TS once you are actually seated, except for the bigger signature meals which might take longer.
We have eaten here several times, one of our favorite restaurants. Once we get seated it's about an our. We have waited as long as 20 minutes to get seated though. So I would plan 1 1/2 hours to be safe.
We have never been seated on time at Sci-Fi in all the times we've eaten here. The film loop runs 40 minutes and we are always here long enough to see part of it twice.
I don't think you will have time for Fantasmic because usually people are lining up an hour before it starts to get a seat. If you plan on 1.5 hrs for dinner you will be done if seated on time at 7:30. If it is a busy night you may be lucky to get standing room only in the theatre. I would try to move up your ressie to closer to 5:00 if you can in order to allow time to get in line for Fantasmic.
We ate lunch there last April and spent at least 1.5 hours there. The meal was served very quickly (our entrees arrived before our starters were even finished), and desserts arrived soon after we'd finished up our main course. However, we checked in 10 minutes before our ADR time (as suggested) and waited for just shy of an hour for our table.
We really enjoyed our meal there though and would really like to go back on our next trip.