There is no need to buy too soon. That is a mistake that will limit your growth. Why lease 8 planes and own one when you can lease 20 planes for the same money?
As long as you will use the planes efficiently, leasing is the way to go (at the beginning).
But when you are generating cash flow, the only way to make you BK proof is by owning your planes..... there is no science involved. If you own your planes, you can always compete heads up in routes without loosing your shirt.