Leech
Life leech or mana leech restores over time a percentage of damage inflicted to life or mana respectively.
Contents 
Mechanics
The total amount of life or mana leeched is a percentage of the damage inflicted and is restored over time. This is different from gaining life or mana on hit, which restores a certain amount instantly.
Recovery rate
Life leech has a fixed recovery rate equal to 20% of the character's maximum life per second.^{[1]} Similarly, mana leech has a fixed recovery rate equal to 12.5% of the character's maximum mana per second.^{[2]}
The leech recovery rate factor is therefore 0.2 for life and 0.125 for mana. There are only a couple of ways for this factor to be altered:^{[1]}
 The of Smothering modifier for maps slows life and mana leech recovery rate by 50%.
 For life leech by a skill supported by the Life Leech gem, the recovery rate is increased by 1% for each point of quality.
The leech recovery rate r is given by
 r = p * q
where p is the character's maximum life (for life leech) or mana (for mana leech), and q is the recovery rate factor including any modifiers.
Example
If the character has 1,000 maximum life and leeches life with the default recovery rate factor of 0.2, that life leech will always restore 200 life per second.
 200 = 1000 * 0.2
Recovery duration
The leech recovery duration in seconds t is given by
 t = l / r
where l is the total leech amount, and r is the recovery rate.
Example
If the character has 1,000 maximum life and lands a single hit that would leech a total of 600 life with the default recovery rate factor of 0.2, it would take 3.0 seconds for that amount of life to be restored.
 3.0 = 600 / (1000 * 0.2)
Multiple leech effects
Separate applications of life leech do not stack cumulatively. Each life leech effect remains present on the character until it runs its course, but only one with the highest recovery rate will actually restore life at any given moment. The same is true for mana leech.^{[a]}
Example with successive single hits
Assume the character has 1,000 maximum life and lands a single hit that would leech a total of 300 life with the default recovery rate factor, then after 0.5 seconds the character lands a single hit that would leech a total of 600 life.
Using the formula for recovery duration, the first leech effect would last 1.5 seconds, and the second would last 3.0 seconds. The second leech effect starts 0.5 seconds after the first, so the total duration that any leech recovery would take place is 3.5 seconds. Both of these leech effects have the same recovery rate of 200 life restored per second, which means neither leech effect takes precedence over the other. Because only one leech effect can restore life at any given moment, a total of 700 life is restored over 3.5 seconds.
 3.5 * 200 = 700
Example with an area of effect hit
Assume the character has 1,000 maximum life and lands an area of effect hit that would leech a total of 600 life with the default recovery rate factor, and that four enemies are hit simultaneously.
Using the formula for recovery duration, each leech effect would last 3.0 seconds. All four leech effects are applied at the same time, so the total recovery duration is 3.0 seconds. Therefore, 600 life is restored over 3.0 seconds.
 3.0 * 200 = 600
This demonstrates that the total amount of leech recovery is not affected by the number of enemies hit.
Example with differing recovery rates
Assume the character has 1,000 maximum life and lands a single hit that would leech a total of 800 life with the default recovery rate factor. After 1.0 second the character uses a skill supported by a quality Life Leech gem and lands a single hit that would leech a total of 400 life with a recovery rate factor of 0.24. Using the formula for recovery duration, the first leech effect would last 4.0 seconds, and the second would last around 1.7 seconds. The second leech effect starts 1.0 second after the first, and ends before the first ends. The total duration that any leech recovery would take place is 4.0 seconds, however the second leech effect has a higher recovery rate and therefore takes precedence during its duration over the first leech effect. As a result, 200 life is restored over 1.0 second, followed by 408 life over 1.7 seconds, and finally 260 life over 1.3 seconds for a grand total of 868 life over 4.0 seconds.
 (1.0 * 200) + (1.7 * 240) + (1.3 * 200) = 200 + 408 + 260 = 868
Sources of leech
Sources of leech include support gems, passive skills, and modifiers on equipment.


These keystone passive are related, but don't add any life leech as a percentage of physical damage.
The Vaal Pact Keystone will allow life leech to apply immediately at the cost lowering it's effectiveness to 40% and regeneration having no effect. Ghost Reaver applies life leech to energy shield instead of life.
Name  Bonuses 

Vaal Pact 

Ghost Reaver 

Notes
 ↑ This is a change from previous leech mechanics, introduced in version 1.1.0. Prior to this change multiple leech effects would queue up, similar to how using flasks works.
References
 ↑ ^{1.0} ^{1.1} Mark_GGG (March 4, 2014). "Leech: GGG clarify please!". Official Path of Exile Forums. Retrieved March 6, 2014.
 ↑ Malice (November 13, 2011). "Mechanics Thread". Path of Exile Forum. Retrieved March 8, 2014.