Depends on if you read what the NPCs tell you to do and where to go. If you do you can probably complete all 4 books and both routes for them in under 10 hours if you're reading fast and going right to where you need to go and doing exactly what you need to do. If you take things slow and read all the dialogue and do all the extra stuff the game will last longer.
If you don't read what the game is telling you to do and you skip all the text you're going to have a bad time, miss out on a lot of the humor and pretty good writing for a WEG, AND you're going to miss all the instructions the NPCs tell you. That's going to cause progression to stop and you're going to get stuck. If you do get stuck I recommend using the search function built into this forum first. If you have a question about this game I guarantee it's been answered already and you can find the answer in this thread.
