Manual:$wgCookieSetOnAutoblock

Details
The cookie that is set will expire when the block expires or after 24 hours, whichever is shorter.

This means that cookies for indefinite blocks will expire after 24 hours.

The cookie is named 'BlockID', prefixed with .

Its value is just the hashed block ID and no other data.

If  is set to false, no cookie will be set during the creation of an autoblock.

See Autoblock for more information.