10 days. If you have a friend (or a "group" that you joined) you can use end tables, wardrobes and cupboards there as storage which WON'T disappear, as long as it doesn't say "steal from" in red on the thing when you "highlight" it.
If you met someone in Riverwood who agreed to help you, they are your friend. Thus, you can use any cupboard, wardrobe or end table in their house to store your stuff in forever.
That said, just go outside of their house SAVE.. then wait eleven days. If it's still there, you know it's a safe place to keep your stuff. If not, you should reload and try a different container.
There's quite a few around, believe it or not.
