The deadline for Form 3520 is April 15 for most individual taxpayers. If your deadline is extended to file your federal Form 1040, then it is also extended for Form 3520, even though it is not filed with your federal income tax documents. Technically it is due the 15th day of the 4th month after the end of your tax year, but the overwhelming majority of US taxpayers are on a calendar year filing basis.
This includes timely filed automatic six-month extensions, two-month extensions for taxpayers abroad, and extensions granted by Congress due to natural disasters or other declared emergencies (usually! You would need to check the rule each time one of these extensions is granted).
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