Manual:$wgDefaultBlockExpiry

Details
Default expiry time in strtotime format, or "infinite" for an infinite block.

It was removed in 1.15, shortly replaced by $wgBlockExpiryOptions in  which was later removed in .

After a request in 32178, this feature was [https://gerrit.wikimedia.org/r/#/c/10469/ reintroduced] in the form of a system message in 1>Special:MyLanguage/MediaWiki 1.20|version 1.20.