How often should I send out an invoice to the client? What is standard practice?
Rule of thumb: One invoice a month.
But it depends on your business model.
Monthly subscription: Invoice monthly of course. Although here you would probably be taking payment in advance.
One-off job: Options here depending on the size of the job. 1- Invoice for deposit. Invoice on completion. 2- Invoice for deposit. Invoice half way through or on milestones. Invoice on completion.
Ongoing work. Options again... 1. Invoice monthly amount 2. Invoice when amount owed reaches a certain limit or a particular length of time has expired.
Unpaid invoice. 1. Send reminder after 2 weeks 2. Send payment required now after 4 weeks 3. Send final demand after 6 weeks 4. Send a letter of intent to prosecute after 8 weeks
But I would have to ask why you are in a situation where your client does not expect when he has to pay. You should detail payment dates in your contract/quote and in your terms and conditions you should lay out penalties for late payment. In the UK it is customary to charge bank base rate + 5% interest monthly on late payments. At this rate it is enforceable by debt collection via the small claims or civil courts - as long as you have a signed contract stating all this beforehand.