"He translated the whole book in three months!"
Translation:Li tradukis la tutan libron en tri monatoj!
A rule laid out by Zamenhof himself about prepositions governing the nominative. Any clause that follows a preposition (such as en, sur, ktp) is in the object (nominative) form, unless there is a further use of direction involved.
For example: La kato kuras sub la tablo, the preposition sub governs the nominative, hence la tablo." But if we want there to be a stronger sense of direction we might say the same sentence with sub la tablon which would mean that the cat was elsewhere and ran under the table.
I cannot think of a way to adequately translate en tri monatojn since that would indicate that the book had, somehow, physically entered into three months.
I do hope that this helps.