That's hard to say, as it depends a lot on the player, settings, and difficulty level. Against 9 Tough AIs, I've found games lasting about 10-12 years at maximum, rarely ending before year 8. I've never won a conquest victory on Immense, though, so I'd imagine that might take longer.

My games last around 15-16 years.

That is at least 700 turns!

I had the maximum of all things, number of minor races, major races, plantes, habitable planets, LoL, everything, and mine lasted to around 50 years and crashed, lost my save of the game, and I was disappointed,    it was an epic game, the Thalans were too tough to be an enemy, cause they wouldve won by the time I had 60 turns, I only had conquest victory on, every other race was GodLike, I wish not to do this in Metaverse, I owned half the galaxy at the time, the other half had Korath, Krynn, Iconians, Drath, Drengin, and the toughest, Terran, but the Terran only owned 4 planets, and they were tough, the rest had from 20 planets to 50, I am the only one that destroyed any race in the galaxy, I destroyed all minor races, and Killed the Altarians, the only race that died, the rest surrenderd.   Everyone used the largest ship they had, with they best weapons they had, I used the best weapons, largest ship, and had fleet ship stuff added to them, had the full tech tree before anyone got fully realized before me, that was tough, I should of shut up long ago in this reply. Hope you like what I said here. The battle of the galaxy was so intense, everyone felt like they were alone and the rest of the races were one as the Dread Lords creators, ain't that the Mithrilar? Well like they were fighting the Mithrilar alone. LoL, they wouldve been so though, noone wouldve one if they only controled one planet. One more thing, there were a total of 2000 Terror Stars in the game, most used on the Terrans, Over 1900 were used on the Terrans, I was playing the Thalans. I'll stop talking now.