You can set up payment terms both on your profile and on your customers profile.
Default company payment terms
You can select your default payment terms under the Settings of your Company details. We default the terms at 30 days but you can customise it as you prefer. When creating an invoice these terms will be applied automatically to it unless specifically changed or your customer terms are different.
Customer payment terms
You can set up payment terms when creating a customer. The terms you select on the Customer profile will automatically apply to any invoice you create for that user. To change the payment terms just go to the customer profile, click on "Details", click on "Edit Customer", select the payment terms and click "Save".