PsyCoil said:
Where was that mentioned?
It's never directly stated in the narration but you can deduced it thanks to two hints:
1. At the beginning of the chapter ten of TLoL, the biography of Estella Congreve, a prominent person on the Nilfgaardian court and close friend of Emhyr: it is stated she outlived him even though she knew him since he was young, ergo she must've been significantly older than him.
2. Morvran Voorhis, successor of Emhyr, is mentioned in the second book to sentence and imprison Peter Evertsen, the high treasurer of Nilfgaardian army (the beginning of the chapter fifth, a segment from encyclopedia). Since Evertsen was already working at this position during the second Nilfgaard invasion (and he is even included in the book), we can fairly assume Voorhis was crowded fairly recently after the war, which means Emhyr could not live and rule for so long. What is the most important though is that Voorhis candidature was proposed by conspirators - he was not a natural heir to Emhyr. Seems like they succeed in installing their own emperor after all.