I did business with a department which was in the process of spinning off from a big Fortune 100 company. They had plenty of funds, but there was a lot of bureaucracy during the transition. As such, they told me they 'required' net-60
I asked for clarification, and they were apologetic about it. They could only run payments on a 60 day schedule. Solution: I billed net-30, 15% monthly surcharge for late payments, which they happily paid.
The lesson was that sometimes a client has no choice but to drag their feet, so be sure to give them that option (for a fee).