Ideally, a contractor should invoice as soon as he has completed the job. However, if he's a sole trader he may only do his paperwork, monthly. He has effectively given his customer a month's credit, so it should be paid immediately.
When my late husband and I ran our own business, we invoiced straightaway through a factoring company which, although it meant we lost a % of the invoice value, it kept our cash flow ticking along, enabling us to pay wages and buy raw materials.