You can build these tents pretty much everywhere except on battlegrounds, capitals, instances and few other specific areas like that.
Tents last for 5 minutes. If you spend at least 10 seconds resting you will become well-rested and gain 10% to all stats for 1 hour. (5 Min Cooldown)
The item disappear after it’s being used. The tents are sold by a leatherworking follower working on a level 2 tannery. You can also purchase these tents by visiting a friend’s garrison with a tanery level 2. Don’t forget a following needs to be working on the tannery.