Hiya, we were there approx 2hrs but really could've happily stayed longer. We were only in Nagasaki for the day (cruise) and spent longer than we anticipated at the Peace Garden & museum area on the opposite side of the city. Only regret was only having the single day there though, Glover Gardens are lovely and Ground Zero area truly awesome. Enjoy.

Yes to both . Definitely you don't need a tour guide . It's not too far away from the cruise terminal. Grab a taxi and you should be there within 15 minutes . When you are done with it and if the weather permits, you can consider walking back to the cruise terminal .