We typically aim for 7:20 - 7:40. That maximizes the chances of getting a table with a view of Illuminations.
Many folks recommend, however, making reservations much earlier, and ensuring that you're done with dinner by 7:20 - 7:40, and then finding a spot to watch Illuminations elsewhere around the World Showcase Lagoon. That's because there is no way to guarantee that you'll get a table at Rose and Crown with a view of the show. They don't allow folks to reserve, or even to request, those tables. Tables are offered on a take-it-or-leave basis -- they won't allow guests to wait for a table with a view of the show any more (because they've had problems between guests before based on perceived favoritism). So each guest booking reservations around 7:30 has about a 30% chance of getting a table with a view of the show, and those odds aren't good enough for many people.