Hi Nancy:
Character breakfasts are a wonderful way to start your Disney day. I find that there is no one set time for all character breakfast ADRs. The schedule will depend on how fast you eat, the location of the character breakfast, and the park opening time. For example, if your desired character breakfast is located inside of the park you plan to visit, you can schedule your meal a bit later than a breakfast outside of the park since you won't have to worry about a post-breakfast commute.
What works for my family is to try to schedule character breakfast ADRs so that we arrive at the parks at park opening. On our last trip, we had an 8:15 ADR for Playhouse Disney's Play 'n Dine at Hollywood & Vine inside Hollywood Studios. We were comfortably finished with our meal and ready to have fun right at the park's 9:00 opening. Because we were already inside Hollywood Studios, we were able to walk out of the restaurant and directly to Toy Story Midway Mania ahead of a large portion of the crowd.
Of course 8:15 may not be the right time for every character breakfast, and perfectly timing meals is a difficult, if not impossible, science to master. If your family is anything like mine, you may have a child who decides to pour apple juice all over himself at breakfast and need an unexpected change of clothes. Or you may decide last minute to linger at the table over some coffee. Indeed, on that same trip, our 8:15 ADR at Chef Mickey's at the Contemporary Resort didn't put us inside the Magic Kingdom until after 9:30.
I would start planning by checking park calendars
here. Click on your desired park and then scroll down and click on the park's calendar to confirm park hours, including any park Extra Magic Hours. You can then make your advance dining reservations online
here or by calling (407) WDW-DINE.
Enjoy your vacation.