Generally, I do not hold a spot for any length of time. If it is open in Sept. but a family does not want to start until Dec., they will pay full fees for that 3 month period. They will usually visit frequently to acclimatize their child. The position is theirs, so they are welcome to drop in any reasonable time of any day they wish.
If it is a family that has a first child with us and who have been very reasonable and kind, I would compromise. They would pay for 1.5 months extra, and I would give up the income for that 1.5 months. BUT I never lose track of the needs of my family and the bills that await at the end of each month!