I think as a general rule, you could say the bigger the company, the longer the payment takes to get to you.
Normally, now though, when I take on an 'invoice account', I'll find out the name, telephone number, and e-mail address of the person who actually makes the payment.
I've got one that can take upto three-months if I don't chase them up, but normally within a week or so of an e-mail, I'll get a reply saying that payment has been made.