33,254 Years

From Srafopedia

The age of the trepanned skulls Lyra encounters in our Oxford. The skull Lyra observed was thirty-three thousand two hundred and fifty-four years old and was that of a sorcerer who wished to use the hole to let the gods into his head. They in fact do allow more Dust to surround the skulls than those who were struck by arrows. This information comes from the Alethiometer. 33,000 years ago is the approximate time that Homo Sapiens Sapiens is believed to have evolved - in other words, the origin of humanity in its current form, more or less.