Manual:$wgCdnMaxAge/en

Details
Cache timeout for page view caching, will be sent as s-maxage (without ESI) or Surrogate-Control (with ESI). Without ESI, you should strip out s-maxage in the Cdn config. 18000 seconds = 5 hours, more cache hits with 2678400 = 31 days.

This variable was renamed from in MediaWiki 1.34.