You can change your default payment terms by clicking on the Settings link at the top of the page. Then, hit the Invoice Details button in the sidebar on the right. Observe the pulldown menu labeled Payments Due. Here you can choose from: immediately (payment due tout de suite), NET 15 (payment due in 15 days), NET 30 (payment due in 30 days), NET 45 (I think you see the pattern here) or NET 60.
Hit the Save Settings button at the bottom of the page when you’re done.
Of course, you can always change the payment terms for any particular invoice on the invoice itself.