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.