The credit period that you state in your policy will be the length of time you allow between the date of purchase and the date that payment is due. Standard practice for small businesses is a 30 day payment period, which will help you to manage your cashflow month by month, keeping your payable and receivable accounts in balance.
Establishing a credit period with your customers should always be done in writing. When they first apply and are approved for a line of credit, you can provide them with a set of terms and conditions in line with your credit policy, which should include their credit period. Also, when invoicing their account you should always specify on the printed invoice how long the customer has to provide payment.
You might consider either offering customers a discount for settling their accounts early, or imposing a fine for late payment. If you decide to do either of these, ensure that they are clearly stated on your printed invoices, and on your terms and conditions if you provide them.