When does the IRS notify you if you are going to be audited?

Generally speaking you will be notified of an audit between 12 and 18 months after you have filed your tax return.  The statute of limitation is 36 months from the time you have filed your return.  Keep in mind, there are no statute of limitation of you did not file a return. 

Note:  If you have understated your income by 25% or filed a fraudulent return then the 36 months is extended.  In the case of the understated income the extension is to 6 years.  In the case of a fraudulent return there is no limit, but generally the IRS won’t audit anyone after 6 years.  (NOLO Tax Savvy for Small Business, p 279)