I realize this is old, but wanted to provide insight. I stumbled upon this by accident in Brisgard and thus did not take full advantage, but in Noarsil I planned it out by carefully studying the Wiki... So..
In Bris, I am CE with several OF troops. I have subs, crabs and eels. I also have a ton of AF surrogates and Tanks. Once I managed to get a quest reward of four FE hover tanks, the rest was easy as those tanks make fighting AF pretty easy. I had to negotiate a lot in later AF and early OF.
That puts the number at four eras ahead.
I wanted to know if it was possible to go even higher, so I was very careful in Noarsil with my tech tree and my quests. I planned it out quest by quest and followed the Wiki at every step. I have managed to be in IndA with three techs left to unlock, and I have 1 PME champ, 2 PME Rocket Artillary and 4 PME Commandos. That is three eras ahead.
However, the last PME quest is to unlock the last two PME military techs, so there is a tech block. I say the max for right now is four eras, but could change if the new era has any quests that pay troops.