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.
Product or service details: Specify Description, Quantity, Rate, Amount and Subtotal. The more details you include on the invoice, the better. This way, your client will know exactly what they are paying for upon receiving the invoice. Make sure that your client can understand every item, so that as soon as they receive the invoice, they can pay you instead of asking questions and delaying the process.