Help:$smwgValueLookupCacheLifetime

is a configuration parameter that sets the cache lifetime of the value lookup cache ("smwgValueLookupCacheType") until it is being invalidated in case this has not happened earlier. It was introduced in .CiteRef::gh:smw:1035

Default setting
This means that by default the lifetime of the value lookup cache is one week until an item is being replaced in case this has not happened earlier.

Changing the default setting
To modify the setting to this configuration parameter, add one of the following lines to your "LocalSettings.php" file after the  call:

This means that an item is kept until it is being replaced, flushed, or dropped.
 * Disable forced cache updates

This means that an item is kept for a maximum of one day.
 * Shorten the cache lifetime to a day

This means that an item is kept for a maximum of one month.
 * Extend the cache lifetime to a month