2-4 hours would be a good value IMHO. For the usage scenario you describe, I doubt you'd be accessing it multiple times spread across the whole day (or at least very infrequently), so 2-4 hours would ensure it gets spin down at least for the night, and remain spun down until the next access which, I would assume, would most likely occur the next evening at the earliest.
Asuswrt-Merlin: Customized firmware for Asus routers
Github: github.com/RMerl - Twitter: RMerlinDev
See the sticky post for more info.