I'm not too sure what it depends on, but in Feb I made an ADR for DD and I and for that day 8:30 was the earliest...we had to get 8:35.
This was a morning EMH for Epcot and they opened at 8:00 so we could not get into the park early and had to wait a few minutes for them to get everything situated before they opened the restaurant and started seating everyone. It was a nice breadfast though.
both are for 8:00am. I made them at the 90+10 day mark. One is for the 5th and one for the 6th day of our trip. They are on a Saturday and a Sunday. I don't know if it's different for weekdays.
My 90 day window started March 2nd, and I just made the reservation on a whim on March 21st after my youngest and I decided we wanted a princess breakfast - no one else in our family does! We did CRT last time we were at Disney so we are going to try this Princess breakfast this time!