I was reading through the TES timeline on the wiki, and I noticed that each of the eras keeps getting shorter and shorter. The first era was 3 thousand years, the second era not even a thousand, and the 3rd era shy of 500 years.

Not sure what to do with this information...just seemed odd.

It doesn't really mention how many years the Merithic era lasts.

Mertheric for the most part took place before (Human) recorded history, so its not surprising that it lasted for a unspecified amount of time.

The era's (Excluding the Dawn and Mertheric) are more based out of, mostly Imperial, political regimes, and the death and establishments of certain dynasties would probably christen their own era to know whose in charge. Or at least that's what Tiber did. The Thrid was rather short since it was the only era that seemingly depended on just the Septim dynasty, and Potentate Ocato declaring the beginning of the fourth right then and there would probably have something to do with it.