50% up front, 50% within 10 business days (2 weeks) of completion.
In your case, it's a month-to-month thing, so if their payment came due on the 12th of Jan, I'd give them a grace period of 1 week. I'd send a reminder on the 13th, shut it off on the 19th and don't contact them until they contact you.
Also: In your specific case, I'd write them an email (or call, fax, whatever) right away and tell them they have 48 hours to get a check to you or their website is shut down. It's been long enough.