What does <none> means for "Minimum cache lifetime" field in admin/settings/performance page?

The minimum cache lifetime is the minimum amount of time that will elapse before the cache is emptied and recreated, and is applied to both page and block caches. A larger minimum cache lifetime offers better performance, but users will not see new content for a longer period of time.

You can read none as 0 in this case i.e. the minimum amount of time that will elapse before the cache is emptied is 0 seconds.

Does that answer your question?

- Arie

Seems to me the cache is triggered on cron job; cron checking re date/time the page can expire. If cron job is after this date/time, the cache is deleted. [I was fuzzier re this till just started using Boost module.]

If so, would seem wise for all sites to have minimum time that's not 0 secs. Especially if have frequent cron jobs; as it's otherwise frequency of cron jobs that determines how often caches cleared.

What I'd really like would be indefinite caching with pages that stay unchanged (esp using Boost module; they can become essentially static, albeit with some changes in Blocks such as for random images).
- and, when pages edited, have comments added or new content should be inserted into pages (such as teasers of new articles, on front page), cache deleted for them during next cron job.
But, I haven't seen this is possible; so min cron lifetime something of a balance between trying to ensure a fair number of anonymous visitors - inc search engine spiders - arrive at cached pages, and fresh content becoming visible.