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I will say that smaller but more consistent upgrades will be better over the long run. It will cause you less fatigue, you would be able to get more feedback, ect. 

How often and how much is something you have to decide. There is also the issue of pacing depending on how you decide to do things.

Do you keep in mind the "lewd" versus the "plot" ration in mind? How do you decide how much of each there will be for any given upgrade? Lastly, overall do you think the game will have more "exploring the LI as characters" or "the MC progression the story on their own" once it's done?