How Does Lifesteal Work?

Please can someone tell me how this works because i have weapon with lifesteal and it doesnt aply it what so ever, the thing is: i am an assassin and have 2 weapons in my hotbar ( one with lifesteal just to heal some health back after a fight and my only lifesteal in my inventory, and another one just to deal damage) .
But the lifesteal only works sometimes and i dont get it, i know that it only goes x/over 4s but thats not the problem, the problem is it aplies like only 1 time (but it heals the full 4s in 1s, what i dont understand either) in about 20 seconds or something like that and thats pretty annoying, i was told to use my multi hit to get the most lifesteal out of an attack but this doesnt work either and thats why i am asking how lifesteal is working.

Maybe this will help:

-Helmet: Vension
-Chestplate: Bobs battle Chestplate
-Leggings: Morph Iron
-Boots: Morph Gold
-1st accessory slot: Ehwaz
-2nd accessory slot: Wastelands
-3rd accessory slot: Morph Amethyst
-4th accessory slow: Morph Ruby

and i am level 66 (by the moment when i am writing this)

 

lifesteal is actually a chance to get some hp back upon a melee hit, but it roughly averages to that amount of hp back every 4 seconds (if you keep meleeing)
ls will not activate with any spell, only melee