This is a must see when you visit Florence. What a magnificant and most beautiful building!!!! The outside of the building was more spectacular to me than the inside. I couldn't take enough photos. The entrance is free but you have to cover your legs. They supply/sell covering material.
The inside of the Cathedral was also beautiful, especially the art work on the seiling of the Dome itself. The other museums and surrounding buildings in this square ,Piazza del Duomo was impressive and worth spending time at. We spend some 2 hours here, just experiencing the whole area. You can just walk 5minutes from the Academcy Gallery to the Duomo.
There are a hop-on-hop-off bus, but the streets are too narrow so it has to travel and stop along the outskirts/sides where all the tourist attractions are. Then you have to walk to and from the attractions in any case. So we were advised by a local to rather walk between the attractions. This was good advise. It was so near each other--most within 5min walk and we could enjoy other thing while on our way.