How old is he ? For how long he is immortal ?

Nobody knows. He's at least several centuries old, if he is the criminal mentioned by the Harmonium officers in the Smoldering Corpse Tavern. Then again, it doesn't really matter exactly how old he is; that's one of the exact details that isn't really relevant to the plot.

Well it's not directly relevant but I'm curious about it. I wan't to know if he is only 70-100 years old, or truly an ancient being that had impact on the history of the planes for years.

Timeless

He can't have become immortal before Ravel's lifetime, but I don't think we know how ancient she is either...

Good question, it took me a while to figure this out.
The first incarnation traveled with Dionarra. Within Sigil, you can meet her father, who still lives. From this, it can be concluded that TNO cannot be *this* old.
Or, was it actually the first incarnation who trapped Dionarra in the fortress of regrets?
Because this somehow contradicts the big number of shadows you encounter throughout the game, where every of them stands for a death of TNO.

Well, we do know that TNO's scarred appearance doesn't necessarily preclude women from finding him attractive (at least if Annah is an indication), so it isn't that much of a stretch for The Practical Incarnation to have been able to flex his massive charisma to manipulate an infatuated Deionarra even after having died enough to be covered in all kinds of scars; the whole 'mark of Torment drawing others to you like a lodestone' bit probably helped to that end as well.

Oh it was the practical incarnation who trapped Dionarra? I wasn't sure about this anymore.

I quote the original Last Rites vision statement on the player character: 'Despite his appearance, he has a raw, unbridled animal magnetism that makes women swoon.'

Practical is one of the last incarnations. I want to say the one immediately before you, but I think Paranoid is after Practical.

Of course, it's quite likely that after Practical and Paranoid made a LOT of enemies, TNO burned through a couple of incarnations REAL fast. Which would explain why Deionarra's father is still around. Imagine if the player incarnation had woken up on the street instead of in the relative safety of the Mortuary.

"Ow, my head... hey what's this talking skull? And these 50 armed men?"
"HE'S GETTING BACK UP! GET 'EM!"

Oh okay so the practical incarnation had somehow found out about the ritual and then how did it find the fortress?

SPOILERS obviously.

"A thousand deaths, and you recover from each. Not so the mind, the mind is much more fragile. Its scars run deep and do not heal." - Ravel

"You have wandered in great circles across the planes in these countless lives you have lived. Such symmetry. Such futility. Go now, back to where it began. Go back, and die." - Trias

The Good Incarnation refers to "thousands" of lives lived by TNO.

"A single memory may increase your character by five levels instantly because he has done everything. He has been around for 10,000 years. He's been a 25th level Mage, and 25th level Fighter, and 25th level Thief, and a 25th level Cleric" - Guido Henkel, Producer

----

The Practical Incarnation claims he was the first to breach the Fortress of Regrets. He also is apparently the one to have saved Dak'kon, imprisoned Vhailor, sacrificed Deionarra, and built the tomb under Sigil. However, he also refers to "the *first* of us" as a separate entity, and he is exposed as a liar as far as claiming to know how to leave the fortress after you merge with him.

My interpretation is that the Practical Incarnation is the one that basically started TNO down the road to this final chapter in his life (that being the game), but was not the first TNO to have existed because when you talk to the Good Incarnation, the game basically tells you that he was the *first.*

The Good Incarnation goes on to say: "If we die, *truly* die, Death's kingdom will *not* be paradise, not for us. If you spoke to these others that were here, know that a fraction of their evil lives is but a drop of water compared to the evil of mine. That life, that one life, even *without* the thousands of others, has earned a seat in the Lower Planes for eternity."

"I became (the Good Incarnation) because for me it is *regret* that may change the nature of a man. But it was too late. I was already damned I found that changing my nature was not enough. I needed more time, and I needed more life. So I came to the greatest of the Gray Sisters and asked her for boon - to try and help me live long enough to rectify all the damage I had done. To make me immortal."

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I always imagined him as more or less "old beyond measure," basically having outlived any non-immortal being who could have known him and his crimes before seeking Ravel.

Good observation.

But here is another thing, how old is Ravel ? Is she immortal as well ?

Ravel is a night hag, which is an incredibly powerful and evil creature that basically rules over the Gray Wastes by virtue of controlling the larva trade according to pen-and-paper Planescape. By convention beings like tanar'ri, baatezu, modrons, angels, etc., are treated as being unable to die from old age, if they age at all. Were I DMing I'd put night hags in the same category. I've been unable to find anything giving a maximum age for them.


“I always saw Ravel as an incredibly powerful legendary figure, so she could have left all sorts of relics, offspring, and perhaps other unfortunate events and historic tales across the planes. She’s made a real mess of a lot of things.” - Chris Avellone, lead writer, invented the character

I believe that like Tanar'ri, baatezu, modrons, angels, etc., Ravel is a reformed soul of a once living sentient being (human, elf, dwarf, etc.). As such, she would be ageless since souls are.