The simple subject is hours, because the simple subject of a sentence is typically only one word and is the simplest form of who is doing the verb. The complete subject would be "Twelve hours" because it includes all adjectives.
The simple subject is "hours" because The simple subject is one that has a single nucleus. The subject's core is a noun or pronoun. In this case: hours. The simple subject is distinguished from the compound subject, which is one that has two or more nuclei.
A subject can be simple even if its nucleus is plural. For example: Children play in the garden. ("Children" is a plural noun, but it is still the only core of the subject, so it is a simple subject).
