Manual:$wgDefaultBlockExpiry

User rights, access control and monitoring: $wgDefaultBlockExpiry
Default expiry time for IP blocks.
Introduced in version:1.2.0
Removed in version:1.5.0
Allowed values:String in strtotime() format, or "infinite" for an infinite block
Default value:'24 hours'

Details edit

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

It was removed in 1.15, shortly replaced by $wgBlockExpiryOptions in r7517 which was later removed in r9748.

After a request in bugzilla:32178, this feature was reintroduced in the form of a system message in version 1.20.