I've done as long as 2 weeks most recently. But it wasn't extremely helpful. I heard that doing it for longer duration is more important. Never did a 30 minute one.
Yeah I can’t speak to the effect of shorter stints. Personally, 15-45 minute practice a few times per week for months (college courses), interspersed with shorter periods of more intense practice (multi-day sesshin) has had somewhat of a lasting or more foundational effect, and seemingly leaving a sort of toolkit that can be tapped into on demand to a limited extent. Overall worth the relatively small time and effort commitment, and probably could result in meaningful long lasting changes in chronic stress. His podcast interview is worth a listen in any case, to get a sense of whether it feels worth a solid try.