that is depends how their account department usually work, some may involve a lots of paper work, some may pay you very fast if not involving large sum of money, you can ask them before you want to take up their assignments.
it all depends on the accounts department of different companies.
You can state 14 days or 30 days payment terms but in actual fact some companies can take 3-6 months to pay. It's hard to say.
Some government takes a long time to pay because they have to go thru lots of departments and paperwork.
Some smaller companies takes a long time to pay because they want to hold on to the cash to "roll"... if they haven't been paid for that particular project, they will not pay you first either.
for eg, company A engages company B to do a newspaper ad campain, and company B ask you to do the photoshoot part. Then company B will not pay you until they get paid by company A. They lay be a long lead time from Company A -> Company B -> you