I don't think is so much an issue for the billing of the claims, as it is if for some reason an insurance company should request a treatment note, and it has not been signed in a timely fashion. For Medicare, they consider a reasonable amount of time to be 24-48 hours.
If Medicare does a ADR before payment on a claim, and the note sent in has been signed a week, two weeks, a month, etc. after the DOS, chances are the claim will deny.
I have billed for several providers in the past where they were horribly behind on signing their notes. Once I explained to them the potential loss of revenue, most of them suddenly found the time to get their notes signed within 48 hours.