When law departments mean what they say, “Law firms, bill us on time,” they generally include in their guidelines something like “Invoices that are submitted more than 75 days after the date of the earlier time record may be delayed or not paid.” Another method to make the point is to take a discount, such as 10 percent, but go ahead and pay the invoice.
Should the discount apply only to the late time or to all the time? I would suggest the entire bill because that creates more clout. You could also have discount tiers, such as 15% for 90 day laggards.
When this technique came up at a recent conference, the law department that applies the ten percent discount said that it did not apply that stick to bills from law firms outside the United States. Billing discipline weakens beyond the borders.