Hi Brandy!
Its hard to say exactly - Do you plan on attending the Mickey's Very Merry Christmas Party on December 2nd? If so, then a 7pm dining reservation might just work for you! However, if you do plan for a fun, sit-down meal during the party, keep in mind that it may conflict with some of the special activities that occur during the party. You won't want to miss the special Christmas parade or Fireworks!
If you aren't planning on attending the holiday party, I would recommend either trying to make your reservation about an hour earlier or switching days so that you aren't feeling rushed to leave the park as the party starts. A 6pm reservation will give you plenty of time to enjoy your meal and then head out of the park before the party is in full swing.
In general, Cinderella's Royal Table can be a VERY popular reservation. If you're interested in dining here, I do recommend securing your reservation 180 days in advance by calling 407-WDW-DINE or by booking
online.
Have fun with the Princesses!