Bring Kleenex. I've been three times and cry every single time in the final room. Allow yourself a minimum of three hours, even if you're not a huge Walt Disney fan (you will be by the time you leave!). The museum is laid out in such an entertaining and informative way, you're just drawn in!
The final gallery is very moving. I appreciate the quiet time it provides after the previous gallery where we go back and experience Walt's passing as the world did in December 1966. That final gallery allows you to collect your thoughts before re-entering present time.