Hello there, @FBW Faith.
In QuickBooks, your prepayment will be posted to your prepayment account (or Other Current Liability). It doesn't post as income, but rather as money owed to your customer.
When the deposit applied is for the full amount of the invoice, it moves from the liability account to your income account. Your invoice will then marked as paid.
About your question, it depends on the invoice date and when you'll have to consider it as sales. It's best so consult your accountant for guidance. You might want to check this link here to find an accountant: https://quickbooks.intuit.com/find-an-accountant/.
I'm attaching these handy articles about customer transaction workflows and manage upfront deposits that you might find helpful. These articles contain details steps and related articles.
Stay in touch if you have other questions or concerns. We're always here for you. Thanks for coming in and have a wonderful day.