There are certain "red flags" that can trigger an audit. For example, operating a home-based business, being self-employed, deducting high amounts for charitable contributions, entertainment expenses, etc.
When the IRS sends you THE LETTER informing you that you are being auditted it tells you the year/s in question. You will also be informed of which specific areas of your return are being questioned.
The amount of time an audit takes and the amount of penalties assessed will vary from one person to the next. As I said, I've undergone two audits. One lasted 4 hours, the other was nearly 6 hours. The first one only cost about $300. The second was more like $1500.
I'm fortunate in that my wife has a background as an accountant, used to moonlight for H&R Block and is a paralegal. (She prepares our taxes.) For each audit she was totally prepared and we just buried the auditor in paperwork. (My wife has a paper trail for everything.)
Three hours into our first audit it began to look like the IRS might wind up owing us money if we kept going. The auditor suddenly offered a lowball figure to settle and we took it. (Just so we could get the hell out of there.) The second one was more complicated and the auditor kept really drilling us on deductions related to our home-based business. She disallowed all the deductions for educational materials and training that we had been taking for years. Again, we reached a settlement and set up a payment plan to handle that one.
Hope this info helps.