Defending bone enchants a bone and last an hour per level unless discharged.
Unguent of Timelessness: When applied to any matter that was once alive, such as wood, paper, or a dead body, this ointment allows that substance to resist the passage of time. Each year of actual time affects the substance as if only a day had passed.
So if I cast Defending bone on a bone then put the Unguent of Timelessness (or the other way around) can I expect that the spell will from my perspective last for months?
Nope, the enchantment is not part of the substance of the bone but a magical effect placed uppon it