Hello, first, good choice in downloading RS1.6, it's worth it even if RS2 is released the day after tommorow.
First, no, in RS1.6, the turns are still 2 per year. I know, it's one of the good things EB has, and I wish RS had it too- family members last twice as long!

I don't know about RS2 though, they may have put 4 turn years in there.
Once you seize the lands the roman empire historically had, there will still be opponents for you- the Germans, Samaritans and Dacians all expand into northers and eastern europe, and by then one will probably have beaten the others and will be a decent foe. Also in the east the parthians tend to cause a hell of an annoyance, what with their HORSE ARCHERS (grrrrr....) It depends really on who ends up owning most of the land in the east- you could be facing the Seluicids, the Armenians, or the Parthians.
There is a max number of turns- that is something that cannot be changed- the game cannot go on forever! However, I think it ends around 120 AD, by our yearing system, so you have about 400 years- or 800 turns, to play. The game map is much more detailed- by which I mean, there are more cities and provinces in a smaller area. The map is more 'zoomed in' than E.B.s, if you like. This means it will take you longer to conquer an area, say, Greece, than you normally would.
One more thing- when RS2 is finally released, you will get a proper challenge as Romans- the team have scripted a 'Roman Rebellion', where some of your legions rebel and you can loose a big chunk of your empire. So look forward to that... when it finally comes.