Each shop has a /chance/ to restock every 5 minutes (for costumes this is the 1 and 6), but that doesn't mean an item will actually stock every 5 minutes.

Basically restocking is based on rarity, the lower the rarity the more often it stocks, the higher the rarity the less often it stocks. Which is why you'll see a lot of items in say the fruit shop (lots of low rarity items), but nothing in the costume shop (lots of high rarity items).

Of course shops with more high rarity items will stock less often.

So though shops are on a 5 minute timer to stock, they might always see an item stock 545 minutes. Since Costumes are mostly high rarities, you could end up going 2-3 restock times (10-15 minutes) for eve just 1 items to restock. While with Fruit, with mostly R5 or under might see 3-4 items restock every 5 minutes.

*Borrowed from the forums, answer by gennie*