I have several books on sale in the ibooks store. A couple of weeks ago I did a minor change to the subject category of all my books. About a week after that I started to notice a drop on sales. I am trying to figure out whether this drop might be due to normal seasonal cycles or it might be that my change got propagated and classified the books in the wrong pile.
I've not seen this change take longer than 48 hours unless there were a long holiday weekend or some shutdown like is typical and emailed/posted months in advance (like December 25 shutdown the last few years past).
If you have a concern, you could always log into iTunes Connect and contact support to check that things are not stuck in your case.