File Download Problem

[Guest]

The File Download Dialog Box just disappears when either the Open or Save
button is clicked on the Dialog Box, after I let the File Download Dialog Box
stay there for more than three minutes.

The File Download works fine if I clicked the Open or Save button within
three minutes of the Dialog Box showed up. I'd like to have the Dialog Box
works, say up to 20 minutes.

I tried the following code, but it doesn't work.

Response.Cache.SetExpires(DateTime.Now.AddMinutes(20));
Response.Cache.SetCacheability(HttpCacheability.Public);

Thank you for any help!

Lidong