How to change relatime access time update interval from 24 hours?

It's immutable in the mainline kernel function relatime_need_update() which checks against the hardcoded value of 24*60*60 (1 day), see the source at e.g.:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/inode.c#n1590