My DS took the Explore test in 3rd grade, and he's taking it again this year, through NUMATS. The most valuable thing to me was the report that NUMATS sends, with detailed information about what my kiddo will likely be ready for in each of the next few years based on how he scored. If you have an opportunity to sign up with a talent search like NUMATS, I recommend that. Otherwise, it was very interesting to see how my kiddo fared compared with 8th graders who took the test (and you'll get that info even if you don't sign up through NUMATS). It kind of helps put things in perspective. Another bonus of taking this through a talent search is that you will get access to the past tests your child has taken as well, so you can compare.
Depending on when you sign up, you will get the location fairly well in advance, so you would be able to check out the school ahead of time (hopefully). (We just got our son's location for next month's test today.) The 3 hours is probably accurate, and the kids are separated from the parents the whole time. There are no penalties for wrong guesses in Explore, so guess away. Also, I recommend doing the practice test they send with your registration, with a timer. My DS did not finish any of the sections, likely because he had zero experience before taking it with timed tests. Note that the science section doesn't really require science knowledge, it's more about data analysis, so basically comprehension.
You know your child best. If you don't think she'll focus, then it may not be worth it to try. It is relatively inexpensive though, compared to other ways to get achievement testing.
