He has 12000 base hp +hp scaling and heals for exactly the amount of damage has dealt to the players.
Unless someone hugs him, the healing will never have any significance at all.
Patch 32.4.1 & Upcoming Events
Yasmule
#42
I guess I have to get actual evidence that he can heal without using lifesteal then, instead of experiencing really long fights where his health bar seems to grow after the initial shrink. The group in question was good about dodging the heal bullets.
Also, he still was able to be dragged into the hallway for safe dmg, letting coffins self destruct. I wonder if that affects it.
Mynamerr
#43
The health bars have been inaccurate forever and if they seem to go up then that doesnât mean its because of healing.
Yasmule
#44
https://webmshare.com/voGPd Doing medium to low dmg per phase causes his hp to return to the same base level. Only with heavy dmg (spellbomb) could I get the lower health bar to appear at the start of the next phase.
Ecookied
#45
I mean I soloed one earlier and barely was hit with Ruthvenâs shots and took about 10 minutes. (On a maxed trickster).
Yasmule
#46
Iâm just saying, this dungeon spawns in the highlands, allows the player to clear it easily, and to visibly damage the boss. But if youâre solo and can only do 3k dmg to him per phase, youâll be there forever (yes I tested it, low bursts of dmg donât register. See pic for how much dmg you have to do to have him ârememberâ it). Seems like a change for the worse, to me.
Seelpit
#47
I thought that was removed completely?
Last time I did one, he didnât seem to turn back at all until the coffins were dead.
Skandling
#49
If you wait long enough the coffins disappear of their own accord, spawning their minions. Theyâve always done so I think, but now if you donât clear them yourself their disappearing lets Lord Ruthven reappear. It is a long time though, and with you needing to do it more times it makes for a way longer fight. I did do it once, but now clear the coffins as best I can if soloing.
Nevov
#50
Ruthven is a lot worse to solo now without a pet to let you tank the minions in the fight. It falls into the âDeca wtfâ for me same as how Cemetery rework went.
For experimentâs sake I just went in a couple of Manors solo, on no-pet 8/8 tops/pyra sorcerer.
First one I tried the circling the room clearing coffins, but because Ruthvenâs bat phase is no longer a fixed time, I couldnât predict when he would respawn, so I kept missing half or more of his short vulnerability because I was not in a position to attack him (as even a single minion can wreck you). Annoyed/bored with this after 5 or 6 times missing it, I switched to standing outside the room, clearing the one coffin, and going in only when he went vulnerable. Took 20 minutes total and I didnât count how many rounds.
Second one I did completely by the exiting/entering the room tactic and it took 10 minutes and 15 rounds. Was predictable, and easy, though this same tactic before the rework would take me 4-6 rounds, so a lot slower. Do not like. 
Though I expect on main+pet I can stay in the room so wonât notice things, itâs harsh on the lower pet players, and why would Deca want to make the dungeon unevenly difficult? I donât get it.
As to the HP bars you mention @Yasmule I think whatâs going on is the bar gets screwed up because Ruthven despawns/respawns, and it resets the calculation each time. The fight has a few phases too, which might mess it up (eg the final phase). So donât trust that because the bar is fuller, his actualHP has raised, it might be an illusion.
Shadowfax
#51
Itâs harder even with a pet if you donât have good enough dps. so players who arenât maxed will have some difficulty. It took a while for me to do on an unmaxed mystic (still had tops).
Seelpit
#54
Yup. There was some funkiness going on with setHP behavior, which essentially put Ruthvenâs HP back at a certain value every time he respawned.
From what I heard, the xmlâs a mess. If only I had the time to give a proper offer to rewrite itâŚ