Packed lunch is a restorative item in Dragon Quest IV. The potency of the item is similar to the Medicinal herb but the party may only get one copy at a time. Beginning in Chapter 3, whenever Torneko returns home to rest, Tessie will prepare a new packed lunch for him the next day. There's no expiration time to use the item.
Beginning in Chapter 5, the Hero's mother will also give him a packed lunch to bring to his father.