I've noticed that in the 1.0 version the Marius event wouldn't happen until I had to build several palaces, but in the 1.5 version with the Extended Greek Mod installed I needed only one. Also in the 1.5 version with XGM, it happened a few years before my first faction leader died, and with 1.0 it would occur after a few faction leaders had passed away.

In short: it seems that the version/ mod you play has some effect on how soon Marius appears.

Of course the mod could have secretly adjusted growth rates. But considering that it was in both the 1.0 and the 1.5 version that Patavium was the first to reach an Imperial level, and that it took about the same amount of turns to reach it, it seems unlikely to me that the Mod made much of a difference.

Maybe it's just something that only happens with my campaigns, or maybe the patch changed something - I don't know.