It is difficult to get long jail time over taxes. The government needs to absolutely prove your state of mind that you knew that you were filing false tax returns showing a lower liability than you actually knew you had, and not just that you used some shady methods recommended by some shady lawyers and accountants to help you pay less tax that you really don't know anything about. People that generally have those people in between can generally show some sort of ignorance, if only feigned, that what they were doing was not legal, and proving that they knew it was illegal generally requires an affirmative statement from them to that effect.
There is a statute of limitations for income taxes on misstatements not arising from fraud, at least federally, up to 6 years if the misstatement was over 25% of total income. There is no statute of limitations on fraudulent underreporting of income, but again, that fraud has to be proven in court. This is the civil standard, not criminal as in the above paragraph, so isn't quite as hard, but it's still hard, often due to lack of evidence.
https://www.forbes.com/sites/robert...n-or-fraud-timing-is-everything/#406a9f182c24