The enforcement period is 6 years but there are facts and circumstances exceptions. https://www.irs.gov/irm/part4/irm_04-012-001.html#d0e61
Also, until the taxpayer signs a return, whether actual return or SFR, the statute of limitations on assessments does not begin to run (scroll down to SFR). In other words, that year is still open.
Substitute for Return (SFR)
The Service has authority to prepare and process a tax return when a person fails to file a required return or files a false or fraudulent return under authority of IRC Section 6020(b). If the Service processes a tax return prepared under the authority of IRC Section 6020(b), the assessment date will start the period for the statute of limitations for collection per IRC Section 6502(a)(1), but does not start the period of limitations for assessment.
If the taxpayer signs a SFR return prepared from income information received from the taxpayer, it becomes the taxpayer’s return per IRC Section 6020(a) and starts the assessment period of limitations.