How does Drupal Cron effect Page Cache?
System module's system_cron() clears expirable entries from cache tables. An expirable cache entry is one that is beyond its "can expire" date, or it's an entry that was initially saved with the CACHE_TEMPORARY flag.

Assuming you're using the default database cache, you can see which entries are expirable by looking in a cache table.  The "expire" column contains the timestamp at which an entry becomes expirable. If the value is "-1", it's CACHE_TEMPORARY and always expirable.

As far as Drupal's default database page cache goes, the "expire" column won't tell you much, since the minimum cache lifetime is not applied to the expire value. Instead, Drupal keeps track of the last time the entire cache was flushed, and will not flush the page cache again until at least "minimum cache lifetime" seconds have passed.