Every quest stage has a different length to it, so a % is hard. Each character has a main line quest (their story), and a primary sex-related quest series. That side quest line can be useful to advance certain things, and change dialogue, so I've included it based on the name/hint given by characters in the game. (e.g. Rae talks about building trust; Caitlin talks about being a good student; Lyriel talks about being a good servant; and Yona wants to be a good wife.)
As of 0.7.19 there are:
- 27 main story quests.
- 27 main line quests for Rae. There are 8 'trust' route quests.
- 25 main line quests for Naomi. There are 3 'femdom' route quests.
- 31 main line quests for Caitlin. There are 6 'student' route quests.
- 30 main line quests for Lyriel. There are 7 'servant' route quests.
- 13 main line quests for Yona. There are 3 'wife' route quests.
- 14 main line quests for Blair. There are 6 'toy' route quests.
You've got a fair while to go before you hit the end of the content.
Just remember that sometimes there are small steps in their quest lines leading to big things. The biggest examples of this, without giving spoilers, are when Rae wants to play with origami, and Caitlin's 'archives' event. (The events themselves are not what they sound like, and you'll know what I meant when you get there.) But the steps leading into them are fairly short. (Rae's origami event is long, and is a trigger for events that just took place over the last two updates, so you will get the idea of small into big.)
If you want to do all of the side content, there are probably another 100+ quest stages amongst the girls and collective household.