You can also use cache macro (see
http://extensions.xwiki.org/xwiki/bin/view/Extension/Cache+Macro) to choose
different timing for each contents.
For what i read, it operates at a differente level, i caches the XDOM, not the rendered
content. I am pretty sure XDOM uses more space in memory that a simple String, you still
have the overhead of rendering operation and you have to manually manage your cache key,
which is far mor cumbersome thant saying xwiki "hey, cache this page for 5 minutes,
ok?) Moreover, documentation suggest to avoid haveing different timeToLive on different
pages, because that's as much threads :s