There are two queues, the priority queue, and the normal queue.
Both queues recache at the same rate. Your recache rate is NUM_CACHED_PAGES / CACHE_EXPIRATION pages recached each day (divide by 24 to get pages recached per hour, etc). So, your cache rate will start slow and ramp up as you add pages.
The normal queue only processes pages older than your cache freshness, ordered by the oldest page first. The priority queue processes pages recached through the API, sitemaps, and URLs added through our site, ordered by the first requested to be recached.
If we recache a page and it returns any of these status codes, we will delete the cached page from our system:
Status Codes on recache that will cause a page to be deleted from our cache
less than 200 greater than 200 and less than 304 greater than 304 and less than 500 3 consecutive status codes greater than or equal to 500
You can read more about Status Codes here