Neo, we know exactly how long each era is. Eras are based off of the Empires of Men.
The Dawn Era, sometimes called the Mythic Era, didn't have time, so you can't really give it any length.
The Merethic Era predated any Empire of Men. It lasted 2500 years and ended with the Alessian Revolt and the founding of the Empire of Men.
The First Era lasted 2920 years, of which 1008 were non-linear time caused by the creation of Akatosh by the Marukhati Selectives (which, by the way, Online goes into a lot). It ended with the assassination of Reman Cyrodiil III by the Morag Tong. The Second Empire of Men AKA the Akaviri Potentate declared the beginning of the Second Era.
The Second Era lasted 896 years and ended with Tiber Septim's rise to power and apotheosis, and the founding of the Third Empire of Men. Elder Scrolls Online takes place around 2E582.
The Third Era lasted 433 years and ended with the Seventh or Eighth Fight of the Aldudagga, where the Time Dragon once again put Dagon back in his place for being a "stupid little f*cker". This ended the Third Empire of Men.
The Fourth Era lasts at least 201 years, probably a lot more than that. It could end with the founding of a new empire. At least once, it ended with the return of Numidium and the exodus of the Dark Elves to Masser.
Eras beyond the fourth are hazy. By one account, the Fifth Era ended with the birth of Jubal-Lun-Sul and Vivec's son, Tosh-Raka, the first of the Nu-Men, Amaranth who dreams Akavir. This most certainly happened, as Tosh-Raka does in fact exist, but then again, why exactly should we trust the word of an insane dragon-tiger time god?
Though judging by the fact that both "Wood Elves" and "Imperials" are in the game... It cannot be that long.
Imperials as a unified people had existed for about 3500 years at this point. The Bosmer had existed for over 4000 years. Not sure where you get the idea that either of these are exactly new...
Also, I never said lore doesn't matter, just that I can enjoy the game *mechanically* despite apparent discrepancies. That said, I find TESO's lore more enjoyable than Kirkbride's C0DA.
Too bad. The Towers are a pretty big part of that C0DA, and none of the games have bought into that story more than ESO. 'Course, you could always just ignore the parts you don't like.