What is the difference in using nous étudiions pendant des heures and nous étudiions des heures? What is the difference in using pendant?
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
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.
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.