I'm with sliepnir on this one. Don't leave client site until you have a signature. The signature is what validates your work, and limits the possibilities of contesting your right to being paid.
As for what duration of credit you give, that is a market decision. I would simply suggest that you not make any pre-imprinted boxes with delays printed next to them, because if a client sees that you admit 90-day delays, they will want 90 days to pay you. If you do allow delay in payment, do it on a case-per-case basis, in your writing. And on both copies.
And get them signed.
Pascal.