@JohnsNotHere: I'd agree with @m4iler 30 days with a late fee. It's a fair compromise that most people will find agreeable. Don't shy away from gentle, then firm, reminders for payment though. I had clients that were consistently late. I worked closely with them and knew it was just poor process on their behalf. After a while a quite chat then a notice of intent to charge late fees soon sorted things out.