How often are they supposed to stock?

EDIT - now it's been 24+ hours for most of them . . .

The most recent restock is currently 18 hours ago (4 shops with another at 19). Accessory bin hasn't stocked in 3 days. Plushie parlour hasn't restocked in 4. The rest mostly last restocked 1 day + ago. Most shops are almost completely empty.

I think I bought around 3 items from NPC shops during my Kumos quests today.