Actually, that's a good question. Times may vary depending on the park hours (or maybe not), but what I've generally seen on Saturdays is that they are fully open for breakfast until 11:00 am, then they shut down the Frontierland side to change over to serve lunch while they continue to serve breakfast on the Adventureland side. I'm not sure when they totally stop breakfast, but somewhere in the 11:30-12:00 range.
That's really good to know - thank you! I am going to have to try to make it there for breakfast some day. Everyone's talked up the Mickey pancakes so much that I need to try them now