What is the average lifespan of witchers?

This reddit posts suggests that Geralt's age is about 100 years.

The age of Vesemir must be at least 150+ (considering the above info), because in the first episode of Hexer, Vesemir is shown as being old (must be 50+), and Geralt seems to be around 13 years old.
Now, considering the timeline of Witcher 3, Geralt seems old, so Vesemir's age must be around 50 years more than Geralt's.

The reason for considering Vesemir's age is that they are both mutants and their ageing rate could be changed, increased or decreased. I can't say anything about that.

However, in Witcher 3 Vesemir dies at about 150 years of age.
So, the maximum age is about 150 (it can be still more if Geralt crosses 150).

Considering Leo's case (minimum), he seems to be at about 20 years old at the time of his death.

So my question is: What is the average lifespan of a witcher?

Please give a good source for your answer.

No Witcher has ever died of old age in the books or in the games. So it is almost impossible to ascertain when a Witcher will die.

Sorceresses can live forever because there is a potion that they can drink that is made by Ortolan that stops the aging process.

You won't find any documentation on the lifespans of Witchers or Sorceresses in the books, so I think this is the best answer I can give.

Neither reddit, Hexer nor Witcher 3 are reliable sources.

The only credible canonical source for Geralt's age appears to be an interview that the author of the Witcher books, Andrzej Sapkowski, gave to a Russian fan site¹⁾, where he states that Geralt is "past 50" during the main storyline of the books. Witcher 3 takes place some 6 years later, so Geralt would be around 60 in the game. And the part in the game where Vesemir says that Geralt is nearly a century old isn't canon.

We don't know how old Vesemir is but we know he is the oldest witcher alive. The books and game both claim that no witcher have died of old age though, so we can't know how old they can get. Vesemir is likely far older than Geralt.

There's an episode in the epilouge of Season of Storms worth mentioning, where the sorceress Nimue is rescued by a witcher looking very much like Geralt - it is almost certainly him even though he denies it himself. This happens 105 years after the main story of the books, in which case Geralt would be 155+ at that point.

Unrelated to the question, we know that Yennefer is exactly 94 years old in the books, so she would be 100 in the game. Ciri is 16 in the books and 22 in the game (but never went through the Trial of the Grasses, so she isn't physically a witcher).

¹⁾. https://scifi.stackexchange.com/questions/151540/approximate-age-of-geralt-when-he-is-getting-ciri