I have no idea. I no longer pay board (I clean stalls, wash buckets and so on) but when I did, whenever my BO got around to writing me out a bill (sometime monthly, sometimes it would be 4 months) I wrote a check immediately so I wouldn't have to worry about it.
I don't think an escalating late fee is unreasonable. You could also suggest to your chronically late boarders that they pay in an "extra" month of board so they won't be late in the future. My mortgage company has a $75 late fee so to ensure I never, ever have to pay it, I am paid up 1 month in advance so even if my check gets in the mail a bit late, it's still not late.