What would happen if a Shinigami with exceedingly long life-span dies for a human? It was said that when a Shinigami saves a human with his Death Note, that Shinigami dies, and his remaining life-span is transferred to the human he saved.

But imagine the following situation, a Shinigami has accumulated 500 years of lifespan by killing a lot of humans. That Shinigami then kills someone for a human, to save his life, and dies.

Does that mean the human would gain 500 years to his lifespan?
Essentially yes, they will become semi-immortal. They will have a long, long natural lives but they'll still be vulnerable to death.