Revenue Recognition in Fixed Fee Contract

Hello All

I have a scenario where I enter in a contract with Customer that is Fixed Fees.

Then I supply Product to Customer Every Month and there is no Subsequent Invoice.

Can any one tell me how to Recognize the Revenue.

Example

Contract - 1200 Dollars. Customer Invoiced and Paid Upfront.

Material is Sent - Every Month (Create Delivery From Contract)

Business wants to Recognize the Revenue when Material is Sent

Regards

Mukesh Mahajan