The ability to process recurring charges or subscriptions using our API is currently available in beta. Subscriptions will allow you to charge customers on a regular basis with out additional input from your customer. If you'd like to implement this functionality, please view the integration tutorial in our Developer's Guide Beta Features page.
Alternatively, you can process recurring charges by sending Google Checkout email invoices to your buyers. To do this, you'll need to send an invoice to initiate each billing recurrence manually.