Today, I rediscovered this nice article about browser cache usage: Performance Research, Part 2: Browser Cache Usage – Exposed!. It gives you a pretty good idea about the average cache usage. Bottom line: Optimize your site for no cache hits at all and you are good.
40-60% of Yahoo!’s users have an empty cache experience and ~20% of all page views are done with an empty cache. … It says that even if your assets are optimized for maximum caching, there are a significant number of users that will always have an empty cache. …reducing the number of HTTP requests has the biggest impact on reducing response time. The percentage of users with an empty cache for different web pages may vary, especially for pages with a high number of active (daily) users. However, we found in our study that regardless of usage patterns, the percentage of page views with an empty cache is always ~20%.