How often do the arxiv moderators reclassify submissions?
As @user2768 points out, we can't know exactly why your article is on hold.
However, there could be several reasons: proper classification, pending moderator approval, presentation issues, copyrighted PDF, etc., to editorial concerns. Sometimes due to oversized submission. You may want to look at arXiv submission status. You can try reaching to arXiv moderators if you need to enquire about this.
As you are asking about experience with long on-hold status: Yes, I have experienced once this status for over 45 days. Note that, I didn't bother asking them either. They automatically posted it in subsequent announcement. But this was more than 2 years ago.