Short answer: Today your age attestation is recorded once against your account and does not expire, so there is no periodic re-attestation prompt. A recurring cadence is planned to arrive alongside third-party age verification; 30 days is the industry default, but the interval is not settled yet.
When the re-attestation prompt appears
It does not, currently. Your attestation is stored against your account the moment you confirm, and it stays valid across both the Vizno and Vizno Uncut fronts until a cadence is introduced.
What triggers re-attestation
Nothing today. When a cadence does arrive it will be time-based rather than session-based, so you would not be prompted on every sign-in - only once the interval since your last attestation has elapsed.
Age-gated content and Vizno Uncut
Attestation applies specifically to content that requires it, including all content on Vizno Uncut. Once a cadence exists, a lapsed attestation would pause access to age-gated material until you confirm again. General content on the mainstream Vizno front that carries no age gate is unaffected.