Hard-coded as a cookie expiration MAXIMUM.
You can still adjust it to 15 minutes, 1 hour, 1 day, or whatever.
As long as you know the maximum cookie expiration time allowed is 1 week
I'm registered with a few sites that have it set to 20 minutes. Very annoying when you're trying to contribute a long post, one that requires research, and you are told that you're not logged in when you go to hit that "Submit" button.
I'm also registered with a few sites that allow you to change the cookie expiration time in your profile settings from "1 hour" / "1 day" / "1 week" / "30 days"... letting the user choose whats best for them. The board default setting is whatever the admin choose in the control panel -- similar to how we currently have a posts-per-page default that can be modified in each user's profile page.
