In most cases, it depends on the restaurant.
Most restaurants, i.e. Signature Restaurants, don't open until 5:30p. In that case they do not serve breakfast or lunch, so that's easy to remember.
The switchover for breakfast to lunch is usually 11:00a, such as Grand Floridian Cafe. More often than not, the last breakfast reservation is between 10:30a - 10:50a. They often do not take the next reservation until 11:30a, which is when lunch begins.
The switchover for dinner is often 3:30p - 4:00p, such as Biergarten.
No one on these boards can give you a 100% exact-science answer. There is none. If we knew which specific restaurants you were considering, it might assist us in helping you to decide.