If you file your income tax returns on time, you must retain your records for six years after the end of tax year to which they relate. For example, you must keep your records for your 2012 taxation year until the end of year 2018.
If you file your income tax returns late, keep your records and supporting documents for six years from the date you filed the late return. For example, if you file your 2007 income tax return in 2010, you must keep your records until 2016.
The six-year requirement also applies to returns for GST/HST and excise duty.