When do pages of the Master Dungeon Guide get filled?

I have the quest "The Misanthropic Jotun Principle" completed in my quest log, but haven't picked up the reward yet. I completed this quest quite a long time ago and have forgotten whether I done the dungeon in Hard mode or Normal mode.

Does the page in the guide fill up on completion of the dungeon or completion of the quest? If the latter, does it matter which mode I pick it up in to fill the page in the appropriate book?

If you complete the dungeon you get the entry. It doesn't matter in what mode you picked up the quest, the mode that you were in while completing the dungeon is what matters for an entry.

Thanks, just what I needed to know

If you have the master dungeon guide in your inventory and the dungeon quest open in your quest log, the page gets filled in the moment the dungeon end chest appears.

If the guide was not in your inventory, get it later and go to one of the four NPC's who hand out the dungeon guides and let them fill the pages retroactively for 100g per entry. This does not always work, though.

Turning in a completed dungeon quest does not fill any pages in the guide.

I believe you have 30 days from completion of the dungeon to get it recorded in the book, if you didn't have the book in inventory at the time. Same goes for campaign story books.

This is correct. So if you didn't have the guide when you completed it a couple of months ago you will need to do it again to get credit in your book. Also each dungeon has a little icon of a cave door showing where it is, once completed a chest appears as the icon instead. This can help you figure out what dungeons you have done or not done.

Also note that once you finish the book you can get a new one and do it again