Q:

decade, century, millennium, what is next?

The title of this question says it all.
Are there nouns for longer periods than a millennium ?
I mean words designating a specific number of years. 
Era, age and epoch don't count as they just indicate a period of time (possibly quite long) that is not counted in years, but that is defined by of a characteristic of that period.
(E.g the "Age of the Dinosaurs" was a fair number of millions of years, but not a specific number.)

A:

There is presently nothing that comes next.
Some terms referenced do have year values assigned to them (Epoch at 1,000,000 years and Aeon at 1,000,000,000 years) but common usage relates to geological time periods which are not consistent in length.
Once beyond millennia we use numbers of years such as "One Hundred-Thousand Years", or some use metric prefixes to 'annum' (for example megaannum as referenced in Wikipedia) but they are basically the same idea.  To follow the same principle from a Latin root form (as Decade, Century etc are latin) then `Decem millennium' (10,000 years) would arguably be closest to our existing words but it would be unlikely to see common use.

A:

The next stop after millennium is terasecond
For rounded number of years, it's megaannum
An interesting table can be found here

