When is Form 5472 due for a foreign-owned single-member LLC?

With the pro forma Form 1120, by the Form 1120 due date: 15 April for a calendar year, or 15 October if you file Form 7004 for an extension first. It is sent by fax or mail to a dedicated IRS unit, not through normal e-file. Keep a record of every transfer between you and the LLC during the year, because the form reports them.

Source: Instructions for Form 5472, Internal Revenue Service