It does not give a precise length of time on their way out, but looks like 17 days based on the conversations. They left on Midsummer's Day. On the way back home with Gandalf he spends a week there.
Rivendell was not central to the story of Bilbo and the Dwarves and the quest for Erebor. Elrond played a minor role in that he deciphered the moon letters but was not part of the quest. Rivendell...
That's easy. Destined. The books go in this order.1. Marked2. Betrayed3. Chosen4. Untamed5. Hunted6. Tempted7. Burned8. Awakened9. DESTINEDI HAVE READ THE FIRST SEVEN BOOKS AND I LOVED THEM. i am...
In general, until it decomposes or gets destroyed, which would be a long, long time. In schools, only about 5-10 years, as books are always getting updated.