The due date for the invoice is totally up to you, however, most invoicing systems are set up on a 30 day, 45 day or 60 day timeframe. You can also make the invoice "Due upon receipt," where the recipient is required to pay the invoice promptly.
What should your invoice include? - A professional header - The first item on your freelance invoice should be your business name, or your full name, in professional (and easy to read) font. The font size should be a little bigger than the rest of the text on the invoice, and possibly even bolded for emphasis.