I would say by Friday.
We were there for 10 and 12 days the past two years with the weekend right in the middle of our trip. It was definitely less crowded Tues/Wed of the week before. The weekend (Sat-Mon) was the most crowded. The crowds lessened after the weekend, but were heavier than the previous week.
Of course, this year it rained (more than I ever remember living in FL) every day of our trip. That really changed the whole atmosphere and my perception of crowds because we weren't walking around and just enjoying as much. Most people were wearing ponchos and just trying to get from one attraction to another.