According to Valkorian the Eternal Empire was his focus, with the Sith Empire a means to an end, considering that his empire was well established prior to Arcann's birth, we can assume the Eternal Empire is at LEAST ~35 years old. Now if we take the apparent age of the Gravestone into account, we're looking at another ~1000 years, which would be about right, considering most living people (according to Koth) believed that the Gravestone was a myth. based on assumptions, we can ASSUME the Eternal Empire is at least 1,035 years old. Zakuul is far too advanced and densely populated to be mere decades old, that sort of infrastructure took a LOT of time to build up