What is the average lifespan of witchers?

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.