Usually you study for hours, so you say nous étudions pendant des heures. The case when some says étudier quelqu’un (quelque chose) is the situation where someone is studying (or doing research) on a topic for instance: nous étudions l’anatomie humaine, la musique, …, of course one might say it in a conversation, but it is grammatically incorrect.

The sentence is at the imperfect tense (étudiions) and is using nous, which means it is formal and literary. In spoken French, the usual equivalent is:

On étudiait pendant des heures. → we used to study for hours, we were studying for hours

or perhaps:

On a étudié pendant des heures. → we studied for hours, we have studied for hours

With some verbs, pendant might be omitted without changing the meaning but it is unlikely to happen with étudier.

On a marché pendant des heures.
On a marché des heures.