The duration that you will buy the resource, in month. It is valid when instance_charge_type is PrePaid.
When period_unit is Month, Valid values: 1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 24, 36, 48, 60.
When period_unit is Week, Valid values: 1, 2, 3.