The reason it takes that long is that they're doing that much testing. My neuropsych evaluation, that lead to my diagnosis, took between 4 and 5 hours. I did get a break for lunch, but other than that it was lots of questions.
As for what its like - answering questions about yourself, doing tasks like connect the dots while being timed, IQ test, answer questions about identifying emotions by tone of voice, and such are the types of things you'll be doing.