If the government owes you, you'll lose your refund if you don't file within 3 years of the April tax filing deadline date.
This means that for tax year 2016 returns, you'll need to file by April 18, 2020 to claim your refund (October 16, of 2020 if you filed an extension).
On the other hand, if you owe taxes, you'll be subject to the failure-to-file penalty, which amounts to 5% of your unpaid tax bill for every month your tax bill remains unpaid after the April deadline, up to a maximum of 25%.
In the meantime, the IRS will send you several reminders to file. If ignored, the IRS may then file a substitute return on your behalf. And while this may sound like a clever way to hand off your tax-prep chores to the government, keep in mind that government-prepared returns may not grant you every deduction or credit you're entitled to.
Chronic non-filers who owe taxes are subject to additional penalties and in some cases criminal prosecution. Delinquent taxpayers who owe more than $25,000 will eventually receive a visit from an IRS representative to collect payment. See the IRS video series Owe Taxes? Understanding IRS Collection Efforts and choose, "4 - Have Not Filed" for more info.