It really depends on your server. In Dec. we had 2 ADRs both for 6.45. The first ADR we got there and were seated early (around 6.15) we had a perfect view of the castle and thought that we'd be able to watch Wishes from the restaurant. However we had an excellent waiter who didn't waste any time getting our food out. We still figured that we'd be able to watch Wishes as we were having a birthday celebration, so we were having a cake. Anyway we were out of there and at the beach well before Wishes started. The second ADR we were seated around 6.40, this time we had no view of the castle and of course, wouldn't you know, we had a much slower waiter, so obviously it took longer! We did manage to get down to the beach for some of Wishes but we didn't see the beginning.
So I would say that your ADR is definitely not too early and if you don't get the really slow server should just be about right!