I've got one customer who routinely gets 60-90 days past due, but they always pay, and I've had them for 15 years and I know they're a single-income family. It's just one of those things that you can't let get to you.
Other than them it seems like the wealthier the client, the slower the pay. That's the thing that gets to be a pebble in my shoe. I've got some dirt-poor customers and average families that make sure the check is in the next days mail. But the doctors and especially the lawyers always seem to play the old "gee, I didn't see the bill" game.