I guess 14-30 days would be about the limit on invoice. After that, dont fly for the operator until paid and maybe tell one or two others that they dont pay on time or at all. That sort of operator might not survive long. I know a couple of such people/companies.
I guess it's a personal choice but as someone offering services in a professional setup, you at least expect to be paid on time and treated fairly.
If you dont get paid and cant afford the fuel to get to work, would you still go?