Furniture (end tables, dressers, cupboards and wardrobes) don't respawn contents and are safe for storage. Barrels, chests, sacks, knapsacks, and satchels will respawn their contents but if you visit the area frequently you'll reset the timer before the containers respawn. Some have shorter timers than others though so unless you test them thoroughly by putting something in them and staying out of the area for X number of days you wont know how many days the respawn timer is set to. Generally it's not a good idea to use respawning containers for storage.