-- "Length of time" is described in units of time.
-- 'Light years' and 'light year kilometers' are both units of length or distance,
so those are not it.
-- "Decades" is a unit of time, but much too short.
Examples: Judaism has existed for more than 300 decades, the USA
has existed for more than 20 decades, and I have existed for more than
7 decades.
The expansion of the universe could be described in units of decades
if you really wanted to, but that would result in a ridiculous number.
It would be about as inconvenient as discussing the expansion of the
universe in units of seconds or minutes.
-- For just about the same reason, "millions of years" is also a too-small unit.
-- When you're talking about stages in the existence of the universe,
the only unit from this list that makes sense is billions of years, or even
multiple billions.