Palermo Stone
Since the cattle count is shown to be regularly biennial during the second dynasty from the Palermo Stone (the year of the 6th, 7th and 8th count is preserved on the document plus full years after these counts respectively), a figure of c. 18 years is likely correct for Khasekhemwy. (or c. 18 years 2 months and 23 days from the main fragment of the Palermo Stone)