How long does a tax evasion investigation take?

Usually tax evasion or fraud investigation requires 1000 to 2000 staff hours. Such investigations usually take 1 year to 2 years. Moreover, it depends on the scale and magnitude of the tax evasion that has taken place. You can read about the Black Money, Tax-Evasion & Tax-Avoidance in the given link.

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