They can start at any time and there are different kind of audits for different purposes.  The IRS has little info on bitcoin I suspect from tax filings as most gains of a material amount happened last year.  Before that it was likely a 'who cares' from a tax perspective.

But there's enough activity at Treasury these days (US) that I have no doubt they are looking into it.  I am surprised that no one here has been contacted or had it come up yet....... but no responses like this to date... and this is the second time we have called for some report of IRS activity.......