"The average sentence length for tax fraud offenders was 15 months." See https://www.ussc.gov/sites/default/files/pdf/research-and-publications/quick-facts/Tax_Fraud_FY16.pdf However I think someone like Manafort would probably have been offered a deal instead of prosecuted to the max. That a-hole Mueller recommended 19.5 years.
Found guilty of five counts of tax fraud, two counts of bank fraud and one count of failure to disclose a foreign bank account. He also admitted guilt of money laundering and bank fraud to foreign lobbying violations related to his work for pro-Russian Ukrainians. Not simple tax crap.