Manual:$wgRCMaxAge

Details
This parameter affects recent changes, related changes, watchlists, and list of new pages, but not history and user contributions pages.

Values at various wikis
It seems that for all Wikimedia projects, and also the Wikias, the value corresponds to one month; at ontoworld.org to ca. 1.5 months. This can be checked with Recent Changes for a not too busy namespace, or with Related Changes for a suitable page.