One cent. It just depends on how you were paid that penny and why.
Generally, if your total taxable income for the tax year was less than $6300 then a tax return is not required to be filed. However, if you have self-employment income over $400, then a tax return is required.
For W-2 earnings, if less than $6300 and any taxes were withheld, then you *should* file a tax return to get those withheld taxes refunded to you.