Whe? 4-5 in a days. I'm in a week.
Just a suggestion, maybe you should concentrate more on your "most loved sketch/concept". Don't think about time of doing that. Actually i got all of my art done in a loooong time. One art can consume more than 8 hours or more. But i love how the process.
Different time used if it was for manga purpose of course.
Hmmm....
Concept is the thing that i draw when i am in the mood of idea, this is an example:
Maybe will do a complete pic from one of them.
Hint: It is weird, but you can play sudoku intensely can improve the concentration 0_0...