600 is about right, i'd think. however, like others have duly noted, it very much depends on what the creator wants in there and a host of other things.
as a caveat, i'd add that if trade is enabled, turn times could theoretically dragggg as the mod progresses. but again, if the intent is to include a set no. of eras and the introduction of particular technologies, then by all means, that is most certainly the creator's prerogative.
now, from my own experience w/ scenarios...which are in essence era-specified mods...due to the sheer length of them, i usually try to trim it down some...or at least try to get it to a digestable amount of turns. and the drop-dead no. for me has usually been about (or to keep it under) 600.
a few other things i've considered wrt the total amout of turns:
Klyden has always reminded me that there are few who will actually get to say, turn no. 550 and so on. and this is a valid point imho. now, this is not to say that folks won't be playing or that it's a diss on the popularity etc. conversely, it's the fact that unless the victory conditions are utterly difficult, vicotry will have been had prior to that and the very back end of a scenario/mod will be rarely seen. and i can attest to this, at least in my own experiences w/ my stuff (and reading the hundreds and hundreds of play reports over the years by our fellow cfc'ers).
lastly, and perhaps most importantly for me as a developer/modder, i take the total amount of turns needed for a scenario/mod and divide it by the number of techs...and toss in a "minimum/maximum" ratio to come up w/ an actual and finite amount. this seems trivial, i realize. however, it's important for me as a developer b/c it can help to allot a set amount of turns to each era and its techs according to the rough estimate of "min/max" times. apologies if it all sounds like a bunch of mumbo-jumbo
but i use this method to nail down my total turn amounts for my stuff and it seems to work well (ie total amount of techs, preferred tech discovery rate).