When do I pay the property fee when I buy an existing house by installment?

It has nothing to do with installment payment. It depends on when the property management company informs you to inspect the house. After receiving the notice, you should go to the site for acceptance as soon as possible. If it is qualified, you should sign for confirmation, and start to pay the property fee next month. If it is unqualified, you can take photos to collect evidence, and then ask the property to be reworked after you sign it, until the payment starts next month.