The duration of Fear is dependent on several factors.
Fear has a potential PvE duration (versus mobs) of 20 seconds.
Fear has a potential PvP duration (versus players) of 8 seconds.
Fear has a damage limit. If the target takes damage equal to the limit, the Fear spell will break, regardless of the potential duration of the spell.
Casting Fear on your minion's current target will automatically cause your minion to stop attacking that target.
Numerous abilities and items such as PvP trinkets may allow players to break the fear effect prematurely, while others may render them immune.
When used against mobs, Fear will cause a patrol to aggro, even when glyphed.
Fear is subject to diminishing returns when used against players. Consequently, repeated use of fear effects upon a player within a 15 second period will reduce the duration of the effect considerably.
Fear is the only fear effect available to players that does not have a cooldown.
Patch changes[]
Patch 9.0.2 (2020-11-17): Rank 2 added at level 52: You no longer suffer spell pushback when casting Fear.
Patch 9.0.1 (2020-10-13): Now learned at level 5 (was 16).
Patch 8.1.0 (2018-12-11): Mana cost reduced to 5% of maximum mana (was 10%).
Hotfix (2018-08-10):Mana cost reduced to 10% (was 15%).
Patch 7.0.3 (2016-07-19): Now learned at level 16 (was 15).
Patch 5.4.0 (2013-09-10): Fear can no longer cause targets under the effects of a movement speed buff to move faster than normal run speed. Additionally, Fear has a new visual spell effect.
Patch 5.0.4 (2012-08-28): Debuff can no longer be dispelled (but can still be removed with immunities).
Patch 4.3.0 (2011-11-29): Spells which have an effect broken by taking damage past a threshold now count damage that was prevented by damage absorbs or other damage reducing effects.
Patch 4.0.6 (2011-02-08): Now has a PvP duration of 8 seconds.
Patch 4.0.3a (2010-11-23): Effects which are broken from taking too much damage now break automatically from about half as much damage taken as before.
Patch 4.0.1 (2010-10-12): Now has a 30-yard range, up from 20. Cast time increased to 2 sec, up from 1.5. Now requires level 14 (was 8).
Patch 3.1.0 (2009-04-14): The damage threshold for Fear to break early has been significantly reduced.
Patch 3.0.2 (2008-10-14): Fear has a new visual spell effect.
Patch 2.2.0 (2007-09-25): Duration against PvP targets has been reduced to 10 seconds.
Patch 1.8.0 (2005-10-10): Pets no longer break off attacks when their target is affected by Warlock Fear.
Patch 1.7.0 (2005-09-13): Will now cause creatures to flee immediately, even if they are already moving.
Patch 1.3.0 (2005-03-07): Mana cost changed to a percentage of base, which results in an increase in cost.
Patch 1.2.0 (2004-12-18): Players now have an increasing chance to break free of the effect, such that it is unlikely the effect will last more than 15 seconds.
Patch 1.1.0 (2004-11-07): Increased the chance of breaking early. Removed facing requirement.
[Fear]. For the manga, see Fear (Legends).
[Horrify], 
[Nightmare]
Patch 9.0.2 (2020-11-17): Rank 2 added at level 52: You no longer suffer spell pushback when casting Fear. 
