I bill most of my clients per search and they pay at the end of the month so unless they have already told me and we've agreed it is acceptable to be a full month in arrears, I consider any I have to send a 2nd month end statement to be unreasonably delinquent. Generally the 2nd statement gets them to pay; however, I have one attorney's office who likes to wait until he closes on the file so they occasionally go longer but his office manager communicates after each statement what the status is and they do eventually pay so I let it slide.