How long does it take for an article to get accepted to the arXiv?
From the arXiv help, emphasis mine:


  On Hold
  
  Your submission was identified by arXiv administrators or moderators as needing further attention.
  
  Submissions may be put on hold for a variety of reasons, ranging from questions about proper classification, pending moderator approval, presentation issues, copyrighted PDF, etc., to editorial concerns. Most of these do not require any further input from the submitter and will be dealt with in due course. arXiv urges submitters to be patient. Due to the large volume of submissions, it may take several days before a resolution is reached.
  
  Note that oversize submissions are automatically put on hold, and this is one particular circumstance in which submitters should follow up with arXiv administrators.
  
  Submissions in this status will not expire. Authors must not create a new submission with the same content when a pending submission is on hold.
  
  Please be sure to include the submission identifier of the form submit/NNNNNNN when contacting arXiv administrators.


In other words: wait a little while. Especially at busy times of year for academics, like now. After a week or so, if nothing changes, you can contact the arXiv administrators following the instructions above.