Monthly on scheduled/regular service, time of service for new one time customers ... I do have a couple of these that call every year for a service or two and I will give net 30 on those. More frequent and people don't know what has been paid and will cause headaches, not to mention costs (office supplies, postage, time, etc). Any longer and you will be set-up for big loses on non-paying flakes. Believe me it happens at some point or another to everyone. One thing I do is give the option of being billed on the first or the 15th of the month. This makes it easier for some people to budget in their payments.