Something like author_first_patchset_creation_date<=90d, as a pseudo query.
Current Last Attracted Developers widget on Community > Demographics dashboard indexes any first Gerrit activity such as adding a random comment, while we are mostly after patch authors. Also, filtering on response times for those very first patches by new contributors on other dashboards currently seems to be only possible by exporting the user names (CSV etc) and constructing a custom query out of them to apply on other dashboards.
That also includes visualization(s) to see retention rates for newcomers (after a first patch, for how long are they around? Percentage of people who stick around?) May need more clarification though...
Non-public upstream support ticket: https://gitlab.com/Bitergia/c/Wikimedia/support/-/issues/82