As best as I can tell from the schedules, Wishes Fireworks are scheduled for 8pm for most days in January with the exception of the first week when it's scheduled for 9pm.
The time of your ressie is mostly just a formality from what I could tell during our last visit. Seating begins an hour before the scheduled time for Wishes. You check in at the podium where they will give you disposable bracelets to wear so that you can exit and re-enter the terrace as you please. Tables are pre-assigned and there will be a name tag with your last name on your table waiting for you, so you don't have to worry about lining up at the podium early in hopes of grabbing one of the front tables. I have heard that the table assignments are based on how early you made your ADR. The first people to make an ADR for a given date will be given the "front row" tables. The desserts and drinks are available in a buffet. There's usually a photopass photographer stationed inside the area to take pictures with the castle in the background.
The people seated in the backrows of tables usually will move up and stand along the railings because the overhead ceiling creates an obstructed view of the fireworks. Not to worry - there's plenty of space for everyone at the railings.
Hope that helps! We had a great experience last time and it was such a nice and relaxing way to enjoy Wishes! After Wishes, we sat around at our table for another 45 minutes to avoid the crowd crush exiting the park - CMs were cleaning up the area but they were perfectly fine with us hanging back.