cache-control max-age

Super Cache sets the max-age of all pages to 3 seconds, which seems odd to me. I’d like it to be much longer, like a day or a week even on sites that update rarely. There is no way to change this within the plugin. I found this thread, but does anyone know a way to do it without editing the plugin; a way to modify the header before the page is sent?

http://wordpress.org/extend/plugins/wp-super-cache/