It goes up to 1E2000.Than 2E400's...Is there certain time limit like 2000 years before the era changes,or is it important events that change the era?

I don't know all the specifics, but it has to do with important events. For example, Tiber Septim uniting all the provinces of Tamriel marked the end of the Second Era and the beginning of the Third.

The emperor usually declares it, I believe. The first era was the rise of the Alessia Empire, it's fall, and the rise of the Reman dynasty. Second era was the rise of the Potentate (immediately right after Reman III's assassination) to the rise of Tiber. The third era was made when Tiber united all of Tamriel to the Oblivion Crisis and the fall of the Septim Empire.

And the fourth was when it all fell apart... Hmmm.

It changes whenever it wants to.

What happened before Alessia & Co.?

Gosh, time keeping is sooooo Imperial-centric.

Before Alessia and co.? Merithic Era, but no one remembers that, or even really recorded it (what a bunch of idiots, if you ask me).

I think it's less Imperial-centric and more White Gold Tower centric. The land and people surrounding White Gold Tower are coincidentally currently named "Cyrodill" but that's just a superficial distraction. I don't think it really matters who occupies the land or the Tower. It's the nexus itself that seems to be tied strongly with major events. That's the vibe I get anyway.

Other peoples probably have/had their own time system. The Yoku certainly did, and I expect the Dunmer had something centering on the rise of the Tribunal. We use the Imperial system because they are the ones controlling the continent at the moment.