Oblivion:Respawning

Respawning is the process that regenerates generic NPCs and determines how often you can harvest ingredients. Respawning may also change the items found in containers and leveled lists. However, there are only a few types of items in a cell that actually respawn. Food is one of those types, and the only type of item which really respawns, after it has been picked up; most ingredients and beverages belong into the food-category. Items, that have been moved around, will return to their original locations. However, items that have been picked up, either by the player or any non-player character, will never respawn, this also applies to items bought by the player. Item, in this context means any movable item, which has been placed into the game via The Elder Scrolls Construction Set, and therefore has not a FFxxxxxx reference-id. Any other item dropped in a cell, including food items, will simply vanish after the cell has respawned.

In this article, generic NPCs, ingredients, containers and leveled lists will be referred to as objects or respawnable objects.

Getting Objects to Respawn
To get objects to respawn, you must leave the area (a.k.a. zone or cell) containing the objects for three consecutive in-game days - specifically, 72 hours from the next whole hour. For example, if you leave an area at 1:05 pm, the objects will first respawn at 2 pm three days later. If you re-enter the area before 72 whole hours are over, the respawning clock gets reset.

For interior areas, you must leave the area (go through a door that triggers the display of a load screen and/or an auto-saving message) in order for the objects to respawn, or - for exterior areas - you must travel at least a couple of areas away or go into an interior area such as a cave, fort, building or city before waiting for the three days.

The majority of containers and dungeons respawn, so it is not recommended you store your items in such containers. All houses have containers that do not respawn once you have purchased the upgrade (or in the case of Benirus Manor, you have to finish Where Spirits Have Lease; before that, the containers can reset, deleting all contents and adding new random ones appropriate to the container).

Safe Containers
Few containers in Cyrodiil are safe (i.e., do not reset). Items placed in a safe container will remain there, even if you do not visit that zone for a long period of time. Additionally, NPC characters will not remove items from safe containers, with two exceptions:

Mazoga the Orc will eat food out of the containers in the White Stallion Lodge. 
There is an enchanted chest in the Arch-Mage's Quarters of the Arcane University. Once you put a compatible ingredient in, after one day 10 more of those ingredients will be added. Remember that although this chest multiplies ingredients, it is not a safe container. After one week everything in it will disappear.

One unique safe container is available for free from the very beginning of the game.  It is a tree stump located in the Market District of the Imperial City, in the courtyard between Edgar's Discount Spells and Rindir's Staffs.
 
There are a few types of containers that are safe, no matter where you find them:

Clams – there are 318 clams in the game, found underwater in various bodies of water. Seven can be found just outside the entrance to the Imperial Prison Sewer, when you first finish the Tutorial. 
Torn Sacks of Grain – labeled "A Torn Sack of Grain", there are 91 that exist in the game. 
Open-topped Grain Sacks – there are 520 such containers, all labeled "Sack". When you first find them they will either contain 5 identical samples of a grain (Corn, Flour, Rice, or Wheat Grain) or be empty. Only two in the game are unsafe; see Containers &sect;&thinsp;Safe Containers for details. A large number of other sacks are also safe containers, but cannot be safely identified visually.
Certain containers (such as dressers and chests) in any house you have purchased, as well as any display cases or open space. This means that if you haven't purchased upgrades to your living spaces, you can store weapons, clothing, items, etc. by dropping them in any open space and they won't be removed.  If something rolls under a bed or cupboard, simply retrieve it by using a telekinesis spell, or by exiting and re-entering the cell until it reappears. Check the individual house articles for details on these containers.

Notes
  You can alter the respawn timing using the console command  , where xx is the number of hours between respawns. The default is 72.