Apparently if you are an Independent worker, and after you have presented your bill to your employer, it is not appropriate French behavior (another "code") to ask when you could expect to be paid. You must just wait until they decide they will pay you. It could be a day, a week, or two weeks. And if no money shows up? I guess you have to wait until you can meet with them again to politely inquire.
* This is how it's working with my husband's employer anyway. Perhaps it isn't like this with every company. But this has been my experience with them. This is very frustrating and makes it pretty difficult to budget, plan and allot expenses.