What is the longest tax evasion sentence?

7 years imprisonment is the longest tax evasion sentence. 3 months imprisonment is the minimum sentence for tax evasion. The sentence for tax evasion is dependent on the quantum of tax evasion. The punishment depends on the amount of tax under reported or evaded. You can read about the Black Money, Tax-Evasion & Tax-Avoidance in the given link.

In the Income-tax Act of 1961, under Chapter XXII, there are provisions for prosecution of people who indulge in tax evasion.

Further readings:

1. Black Money: Definition, Sources & Measures to Curb Black Money. [UPSC Economy]
2. Double Taxation Avoidance Agreements (DTAA) – Countries Involved, Income Tax Provisions