From when is the credit period counted for goods? From delivery to the customer? Or from some other date?

And how long is the credit period for a seasoned reputable builder?

It is often the middle to end of the month following invoice date, so all April invoices payable by xx May etc.  The convenience is often not in the credit period offered but in just making a single payment.

 

For contractors they will invoice the client so they get paid before the merchant.

Just call them up and sweet talk any terms you want 

The main benefit you get from a credit account is the invoicing - I ran a cash account with the merch and it was chaos tbh - switched to a credit a/c and all good.