Question

"An hundred years should go to praise/Thine eyes, and on thy forehead gaze" is an example of

Answer 1

Hyperbole

Step-by-step explanation:

"hundred years" is an exaggeration.

Answer 2

I believe the correct answer is hyperbole.
Hyperbole is a rhetorical figure of speech which show some kind of exaggeration - in this particular example, the hyperbole is found in the words 'an hundred years.' This is so because the poet won't really spend a hundred years to praise the woman's eyes, but is rather exaggerating a bit.