Technically you'd ultimately have to subtract the first ~1,5 to 2 years of development, as most of the content from that time has been reworked or will be reworked at some point and as such was basically done twice.
And compared to your standard (japanese) VN? Well, there you'll have a limited cast of 4 to 6 girls, a small set of poses, a few generic backgrounds and for the most part static sex scenes or maybe a few simple animations. And that's it for the most part. The actual number assets for such a game is indeed quite small. If you compare that to this game, you'll see the different level of effort and detail. As such the time it takes is simply longer, given that everything is still drawn by one person.
The question to ask is simply, how many assets (including the ones to create the animations) can one person reasonably draw per year and that's roughly your answer about the amount of content the game gets.