It depends entirely on the MSC or the work rhythms of the scheduler or the whims of the client (releasing shops to the MSC). Some post 99% of the month's shops between the 18th and the end of the prior month.
Also, many, many shops never see a job board. Make a good rep with an MSC and you should start getting emails about shops before they ever get posted.
Based in MD, near DC
Shopping from the Carolinas to New York
Have video cam; will travel
Poor customer service? Don't get mad; get video.