According to this page, Google requires "as a rule of thumb" that the SXG expiration date is less than 1 day in the future if the content is JS, or otherwise 7 days in the future.
My external Javascript files are immutable. Why should they have a 1 day cache life? Currently I am doing something similar to max-age=3600, s-maxage=604800 for HTML files, and a flat max-age=2592000 for JS.
Is this not ideal?