If I clear a fort, does it essentially become mine? Does stuff I put

[Dreamsxord]

Cleared fort near whiterun. Slept in the bed. Put some stuff in the chest. My fort?

[billyro]

Nope, the bandits will respawn in a few weeks (I think... depends on mods you have) and the stuff in the chests will be gone.

[Deleted472477User]

Nope. when the bandits respawn, your stuff will go poof.

[whateveroo]

Non-resetting containers:

All chests and containers inside purchasable houses.
All containers in the Abandoned House in Markarth.
The chest in the secret room inside Sleeping Giant Inn.
The end table and wardrobe inside the ruined Helgen Inn.
The wardrobe in Riverside Shack.
The end table located in the Alchemist's Shack.
The Apothecary's Satchel located in the cellar of Anise's Cabin.
The Arch-Mage's Quarters after completing The College of Winterhold questline.
The chest at the entrance of Sky Haven Temple.
The metal chest at the top of Valtheim Towers
The ice runner chest in Broken Oar Grotto after the quest.
The chest inside Idgrod the Younger's room, in Highmoon Hall, Morthal.


Any other chest would get reset, takes from a couple of days up to a month for some containers to reset.
Easiest way ofcourse is buying a house and storing your extra stuff there.

[Relativelybest]

Mistwatch doesn't respawn after you cleared it, so if you want your own fortress ruin to safely store your stuff (without having to mod it) that would be the one.

 

Also, IINM, interior cells that respawn do so after certain amount of time from the last point you enter it. (As opposed to exterior cells that always respawn after a certain time regardless.) Meaning you can technically keep any place from resetting itself by visiting it on a regular basis, though if you miss the deadline at any point, all your stuff is gone.

 

I get the feeling, by the way. In Fallout 3 I would always end up living in the Machinist's Forge after talking the Machinist into leaving, because I liked the place way better than the official homes. I was disappointed I couldn't find anything similar in Skyrim.

[gsmanners]

To be pedantic, only player homes get flagged as owned by you. Everything else is either owned by someone else or it's up for grabs. Whether containers in that location reset is a whole other issue, but generally you can count on most containers in an Elder Scrolls game to not be safe.