An employee is considered to be non-permanently employed if he or she regularly works for one or more employers for less than seven calendar days at a stretch (including Saturdays, Sundays and public holidays). It does not matter how many hours are worked per day. Actors who work on a film or theater production for three days, for example, or journalists who write a report in four days, are considered to be non-permanent employees. In principle, they are subject to social insurance contributions, with the exception of unemployment insurance. The employer pays contributions to health, nursing care and pension insurance. This also applies to the non-employment period between assignments - for up to three weeks.

Source: Techniker Krankennkasse (only available in German)