Doki Doki Literature Club Days
A single playthrough does not take long. It is a short vn. Though to see everything it has to offer might take you some time considering that there are random events in it. There is also things you can find/do outside of the game involving the files themselves.
Depending on how fast or slow you can read I would say 1 playthrough could be. Perhaps 2-3 to 6-7 hours?
Doki Doki Literature Club Day 4
Around there anyway. ( I suggest to take your time though since some things happening outside of the text box makes the vn more interesting.