Lessons 6 and 7 tackle things in a different manner to 3 - 5 because they focus on precision and planning compared to the earlier lessons which are more reactive in nature. E.g. If you drew the head a bit too big in lesson 5 that isn't an issue because you can adjust the rest of the drawing. In 6 and 7 however it's much more methodical and requires more planning. In terms of how long it takes to do 1 object construction vs 1 animal construction, I'd say that an animal could range from 1 - 4 hours roughly as it will vary from person to person. A lesson 6 object (orthographic studies + construction) could take up to 2 - 12 hours depending on the complexity of the object + how precise you want to be. One lesson 7 vehicle (orthographic studies + construction) takes around 4 - 24+, again depending on the complexity of the vehicle and how precise you want to be. These are just rough estimates and everyone will take differing amounts of time so take them with a grain of salt.

I would say the difficulty spike isn't that large if you take your time and read through the lesson material thoroughly. I've also recently written a guide for both lessons (which you can find here) which should make the lesson easier if you didn't quite understand the material on the website. Most students are intimated when they see a million lines but when you start the lesson and go through the demos it isn't as bad as it seems.