Is there a set time for how long an era last,or is it just until something significant happens that ends an era and brings in the next.

An emperor tends to call it.
The first era lasted 2920 years, the second lasted ~800 and was started when the Potentate ruled after Reman III's assassination, the 3rd was ~434 years and started when Tiber conquered all of Tamriel, and the 4th started somewhere following the death of Martin Septim and the Septim empire. Generally, a new era wouldn't be called, unless it's the start of another empire and the end of the old ways of the previous empire.

Guess Titus is quite the dude.