Many businesses wait until the end of the month to do their invoicing. Invoice as soon as you have completed the job. If you wait 10 days to invoice then that’s 10 days that you have no chance to get the cash.
Another good practice to get into is to make sure that the customer is happy when you are finished the job and ready for an invoice. You can simply ask them
“Are you happy with the job”
“Can I send you an invoice”
“Ah, yeah. Sure”
This creates a personal obligation and the customer is more likely to pay. Send them an invoice as soon as you can.