The instant a user feels the product's value click for the first time, after which they tend to stick, usually paired with a measurable activation event as a proxy.
Early Pinterest data showed that single-pin users churned, but users who built their first themed board returned. The team rebuilt the onboarding around picking interests and seeing boards populate immediately, which became the standard pattern for interest-based feeds. By 2019 the company went public valued near $13B on the strength of the boards-first experience.