What units are used when describing the length of time that the universe has been expanding? (could possibly be multiple)

billions of years
millions of years
light years
light year kilometers
decades

 -- "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.

therefore, the efficiency is: