The deadline to file tax returns is April 15th of the next year (example: 2020 tax returns are due by April 15th, 2021).
If you are owed a refund (meaning you paid too much tax out of your paychecks for the year), then you technically never have to file; the government is glad to keep your money! But, if you want your money back for a specific year, you will have to do so within 3 years of that tax year’s filing deadline (example: 2017 tax returns are due by April 15th, 2021).
If you owe more tax for a specific year, then the longer you wait to file, the more interest and penalties are accruing. File as soon as possible.
Do your taxes!!!