How often do you rebill?

I have about a half dozen of my own clients, and I send the bill when I send their transcripts.  My quesiton is, when should I rebill?  I have been doing it every 30 days, but I'm wondering when everyone else does it.  I don't want to seem pesty, but I do also want my money.  Thanks.

I have several of my own clients and pretty much know when to expect payment from each of them -- one pays as soon as he gets the transcript, one pays on the 30th day, etc., etc.  I personally do not rebill my clients, but send a friendly email or make a friendly phone call if payment has not come 30 days after it normally does.  On the other hand, opposing counsel gets a bill from me every 30 days -- 2nd request, 3rd request -- and I start calling after 90 days and ask when I can expect payment.