How many hours to block sand?

I like my body panels slick as possible before the primer goes on so it takes time. I don't limit my time and just do the best job I can to save problems later.

The color is going to make a difference on how much time it will take. Block sanding varies by each car and how slick you want it so this would be something that is hard to give an answer on. I had cars that I have worked on cars for a solid week to get them where I wanted them. Black will show every imperfection and white will show the least imperfections.