Now after I said 12 hours, looks like todays book is going 24 hours!
So I guess the right answer for frequency is maybe every 12 hours, maybe 24
And every so often they offer a chunk of 20 books at once!
By the way, there was a dedicated thread few months ago - someone asked help to write same automation script. I also used to run one, but since recaptcha was introduced I don't use it...