The listed duration is the maximum, and Hibernate has a chance to break early, especially on higher level targets.
Tips and tactics
In a PvE setting, Hibernate gives druids a tremendous advantage as it allows you to take on beasts much more powerful than yourself. A good druid can kill an elite beast mob of his or her own level and even one or two levels higher by alternating Hibernate and [Starfire].
Used in combination with [Entangling Roots] and, where permitted, [Flight Form], Hibernate makes the druid an impressive farming class, allowing you to get to herbs and mining nodes without having to fight for them.
Hibernate can be used to help crowd control in instances where beasts or dragonkin are present.
Only one target can be forced to hibernate at a time. Casting Hibernate on a second target will cause the first to awaken.
Like most crowd control, Hibernate doubles in function as a spamable counterspell: Even though the target will not remain asleep while players are beating on it, any of its abilities in progress (with a cast time) will be interrupted.
In PvP, It is good to use against a Hunter's pet, a Druid in Feral forms, and a Shaman in [Ghost Wolf] form. However, due to its cast time, a good player can often shift out of form before the spell is done casting to prevent it from being used on them. It also has a somewhat shorter PvP duration of 8 seconds, and is subject to diminishing returns.