Rule of thumb? Three quarters of law firms bill monthly on each matter?

Recently, at a regional bank’s law department, the billing analyst estimated that 70 percent of the bank’s law firms send a bill monthly. I had thought before then that a higher proportion would hold, but she explained that (1) some firms make a practice of billing quarterly, (2) some firms bill at the end of a case or matter, (3) some firms carry over small amounts of work until the dollars in WIP warrant preparing a bill, and (4) some law firm partners are lazy, slow, or unconcerned about cash flow.