Until Dalinar's talk with the Stormfather, no one (outside of Mraize) seems to think Tezim is anything other than an ordinary human who just happens to be ten shades of bonkers. Which implies that Tezim's rule hasn't been longer than a normal human lifetime.
So what has Ishar been up to in the past few generations, before he took over Tukar? Or do you think everyone just assumed "Tezim" was a Dread Pirate Roberts situation?