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, ...
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.
It is in the imperfect indicative, l'imparfait de l'indicatif. The subjunctive cannot be used without a relative or subordinate clause, usually beginning with que (plus ou moins). Si does not invoke the subjunctive but can invoke the conditional if...then meaning. That being said, si conditional phrases do not always mean the conditional mood.
The sequence ...
This should be the imperfect indicative: it is generally the indicative that appears in clauses introduced by si. In writing, one may see what appears to be the imperfect subjunctive in a si-clause, but this does not occur in everyday speech, and anyway, it is more precisely the pluperfect subjunctive that is sometimes used:
Le plus-que-parfait du ...