What's the ideal length of the sprint review/retrospective based on the length of the iteration?

Question

I've been in several projects using scrum now.

The length of the sprint review and/or retrospective has varied from client to client, from project to project. Some of the reviews became too long (2 days).

Sometimes, I've lost focus while being in this long meetings and just wanted to go back to my safe and happy place (my chair in front of my computer).

What is the ideal length of these meetings? What's the best way to avoid long meetings?