2
1. The stats are still being gathered, they are just not being updated until next cache update - i.e. after 30 minutes
2. Good time periods vary on the frequence of content addition. Depending on the traffic, you can set the caching time high or low. Setting it high will give the best performance, but also means that your site will not be updated that often. For very high load sites, this is perhaps a good trade-off. On the other hand, if you have a site with loads of traffic, it might not matter *that* much if the page load times is a bit higher every 15 minutes or every 30 minutes
