You can agree any reasonable credit period between yourselves. While this is up to you, you may find it difficult to compete unless you offer the same sort of terms as other businesses in your industry.
If your terms and conditions do not specify a credit period, then the credit period is 30 days from the invoice date or delivery date (whichever is later).
There are also rules to protect you against being forced to offer customers unfairly long credit periods. Normally business customers can’t expect more than 60 days, and public sector customers 30 days, unless there is a good reason.