This is the length of time in seconds that a template cache is valid.
Once this time has expired, the cache will be regenerated. $caching must
be set to "true" for $cache_lifetime to have any purpose. A value of -1
will force the cache to never expire. A value of 0 will cause the cache
to always regenerate (good for testing only, to disable caching a more
efficient method is to set $caching = false.)
If $force_compile is
enabled, the cache files will be regenerated every time, effectively
disabling caching. You can clear all the cache files with the clear_all_cache() function, or
individual cache files (or groups) with the clear_cache() function.
If you want to give certain templates their own cache lifetime, you could
do this by setting $caching = 2,
then set $cache_lifetime to a unique value just before calling