How to set Page output caching for some expiry date

Hi,
I am using Page purout cachine in my asp.net web application. I have implemented page output cachine to cache pages at server for 3 days by using the below code
Response.Cache.SetExpires(DateTime.Parse("12/7/2009 11:55:00 PM")); // Consider some feature date more than 3 days.
        Response.Cache.SetCacheability(HttpCacheability.Public);
        Response.Cache.SetValidUntilExpires(true);
        Response.Cache.VaryByParams["*"] = true; 
 
When i request my page first time it is taking some time to return results to the browser and the subsequent requests for the same page is served from the cache and it is fast. But when i request same page after 3 or 4 hours, instead of serving page from server cache it is starting processing the whole page lifecylce (executing stored procedute ...