What is Maximum Lifesteal ?

LS Quints = 4.5% Vampirism + Warlords Bloodlust Masteries = 22% , (x3) = 86%, assuming you count the healing effects as lifesteal Elixir of Wrath = 15%

127.5% healing on your auto's, 30% on all other physical damage, from items, runes, and masteries.

You can go higher tho, technically. Throw in Sion with his passive going = +100% And if your looking for raw healing amount, a spirit visage instead of an additional hydra will boost it more i believe, as will the runic armor mastery.

Could you provide any more context? Are you looking for a particular build that a streamer, youtuber, or pro uses, or are you just trying to achieve the highest amount of life steal possible?

I believe there is no cap to lifesteal.... However I think the absolute max you can get is about 130%....

Olaf Lifesteal runes, warlord's bloodlust, vampirism, runic armor.

Spirit Visage, Bloodthirster, two Ravenous Hydras, Mercurial Scimitar, Botrk,

If not mistaken, bloodthirster stacks, unless that got fixed. If that's the case. Items alone would be 120% lifesteal. I remember a youtuber named Vandiril making 2 graves constantly shoot each other to stack up the E's passive, reason they didn't die was because both had like over 5K armor and MR and because they both had 6 BTs.

But like I said, it could had been fixed.

Strictly speaking about the lifesteal % value, you need to play Sion with lifesteal quints, Vapirism and Runic Armor masteries (new Warlords is not counted as lifesteal). Build one Bloodthirster, a combination of four Death's Dances or BotRKs, and a Spirit Visage. Buy Elixir of Wrath.

Items and the Elixir will give you 95%, plus 6.5% from runes and masteries. Sion's passive adds 100%, so far totaling 201.5%. Finally Spirit Visage and Runic Armor amplify to lifesteal multiplicatively. We multiply 201.5 by 1.25 and 1.08 respectively for our final theoretical limit of:

272.025% lifesteal!

For anyone wondering, Warlords now returns health based on your total AD and not on damage dealt, so it's technically not lifesteal. Before the change the theoretical limit was 339.525% lifesteal.

Also I haven't taken the time to build this in game so the numbers are just theoretical, if someone could confirm it that would be cool.