My dad does auditing and quality assurance consulting mostly in the power industry. And many of those companies are worse then the small fry I did computer consulting for. You can forget getting paid in less then 60 days, the ones that are good with paying their bills time things so the bills are paid 60 to 90 days after invoice.
The bad ones take 90-150, assuming they didn't 'lose' the invoice or something else didn't go wrong. If that happens don't expect to see your money until you hire a lawyer or they need to hire you again.
One company my dad did work for was so bad that the UPS guys wouldn't take anything off or put anything on the truck till they had a check in hand. Took my dad the better part of a year to get his money from them.
Jay