I think "6 days" is a bit of a shorthand we've been using. We don't have a final value, but it might be 160 hours, which is 6 days, 16 hours.
We are aiming to be under the 7-day limit per the CA/B Forum rules about what a "Short Lived" certificate is.
We never want to push directly to the threshold of what's permitted, to allow a bit of margin to allow for a bit of room for whatever potential errors we may make with date math. Call that the fear of another 1715455 - Let's Encrypt: certificate lifetimes 90 days plus one second