If they don't know about it usually there is no limit to how far they can go back, that how it works in most places.
It depends on the source of your income earned/remitted and residence and was it taxable and to be declared?
"Tax investigation and assessment
The Revenue Department is empowered to demand documents and
records for inspection for a period of two years. The period will be
extended to five years if it is found, or there is a reason to believe,
that there was tax evasion, or where a tax audit is conducted for the
purpose of paying a tax refund.
Nevertheless, under the Civil and Commercial Code, the Revenue
Department can assess tax for up to ten years."
To get a more clear understanding of this one needs to refer to the Civil and Commercial Code.