Twenty of anything is called a score. This can be years, eggs, cars, whatever. Although this term is still used and recognized, it is becoming antiquated.
If you're looking for a stand alone word for 20 years (i.e. not including the word years) there isn't one, but a decade is 10 years, so you could say two decades.
A bit more on score:
Score is singular when used for a specific quantity and plural for indeterminate quantities, just like hundred, thousand, million, etc. For example: ten score cars went over the bridge; two hundred cars went over the bridge; scores of cars went over the bridge; hundreds of cars went over the bridge.
There is no special term of a period of 20 years which would be referred to as 'two decades'
You may be thinking of a "score", as in Lincoln's Gettysburg Address ('Four score and seven years ago' means 87 years ago.) It is also called two decades.
It's answered at http://wiki.answers.com/Q/What_is_a_5_year_period_called.
A 13 year period, it has no name
3 decades
8 years.
What is 40 years called
Score
score
A period of 100 years is called a century.
A period of ten years is called a decade. Sometimes a decade is used more specifically, to refer to a ten-year period that starts with a year that ends in zero. For example, "the sixties" or "the 1960s" refers to the years 1960-1969.
There is no single word in English for a period of 300 years. Three centuries. As an adjective, or a noun referring to the anniversary of 300 years, you could use tricentennial.
ten-year
The term is "decade".
A Period of ten years is called a decade.
A period of 75 years is called a "septuagenary."
A period of 10 years is called a decade.
A period of 100 years is called a century.
A period of 100 years is called a century.
A period of ten years is a decade.
The period of 100 years is called a century. A period of 10 years is a decade, and 1000 years is a millennium.
A period of 1 thousand year is called a millennium.
50 years is called a quintuplet
a five-year period is a quinquennium
10-20 years
It is called nine centuries.