I give 14 days credit for commercial. Its on my quote and i also charge late payment. Again its all on my quote. In a previous career i got taken for a ride all to often, the attitude of the big boys pushing the little ones around and living of my money was wrong then an its wrong now.
Most companies are processing invoices on a daily or at least a weekly basis, so there is no real excuse not to pay you. If they dont like your payment terms, they should say so before you start, thats what an agreement is.
A bit black and white i know but it works for me.