It is really very much up to you. We never take afternoon breaks but we only plan a couple of evening events so we leave the parks for our evening meal and after about 1/2 the time. We still take a day break in the middle. However, the last couple of years, the break has turned into not so much break. 17 DD and I hit downtown and DH, DS 13 and DS 8 go to Gatorland.
My only suggestion would be have it 'planned' mid way but the day before, sort of bounce the idea off people. "Tomorrow we are going to stay and swim." If you get 'yippee', you have your answer.