Hey Trish,
I agree with what Tess said. It's mostly what you can handle. At the same time, it may also depend on who you work with. If you are working with a corporate company, they may have a standard waiting period for invoices because they have to process them. For a sole proprietor, it's a little easier to send it earlier.
I work with Coaches who all work from home or a small office that's just them. They sign a contract with me that states they will make payment within 5 days or I may hold their work. After 10 days, I may hold work and they will be charged a late fee. After 30 days, I definitely hold work and they still owe that late fee. That's when I send the first snail mail reminder that is registered mail.
It's a tight plan, but it keeps people paying me within 14 days, which is when (for MY budget) I need to get paid. I use Freshbooks (
http://www.freshbooks.com) for invoicing and I can have 3 reminder e-mails go out after my invoice. I take FULL advantage of that!
Create a great day!
Kristen