In the states eekster, an audit can only be conducted within three years from the due date of the tax return or three years from when you filed your return, whichever is later.
However, they can go back 6 years (maybe 7?) if you understate your income by 25% or more.
They can go back indefinitely if there is fraud. however, fraud is a very difficult concept to prove, it's a state-of-mind.
I've never seen anything over 3 years in my 26 years of practice. I don't think I've even had more than 20 returns audited in that time frame, all of them either very wealthy individuals, businesses or returns with a red flag. If you file a return with a red flag (ie:200k medical expense), you better be prepared to discuzz it. Most of the audits happened in the 80s. I had one client audited in the 90s, the aforementioned red flag. I've had 5 audits this decade, one lasting nearly three years which is in appeals at this moment.
I've never had one run of the mill average tax return audited in 26 years.