I’ll reiterate the earlier points that there is no meaningful average. Every author’s process, piece they’re working on, organization preference, and life situation is unique. Your game could be 150k words and take you 5 years to write from start to finish. It could be 750k words and take you 15 months. It could be started and never finished, or started, then discovered to have so many bugs and fundamental problems that you don’t think it’s worth the effort to fix them all. But if you enjoy your work and if the game stays important to you, you’ll find a way through eventually!
Most CoG games that I’ve edited (more than 10 but fewer than 20, I think, over the years) have been around 500k words after several rounds of beta testing (some slightly less, like around 350k, and some far more, like over 1 million words, but those are outliers), and have always taken the authors more than a year, or so I gather, to write, much less test and edit, which can easily take 6 months or more. Most have had between 8 and 12 chapters, ranging from 10k to 100k words per chapter, with some additional files for startup, stats, and sometimes atemporal scenes or calculations you’d rather keep outside of the chapters.
I recommend buying a few games you enjoy and doing some code diving to see how they’re structured, as it’s quite different from static fiction.
All I can tell you is strap in for a very long ride. From what I’ve seen, there is no “writing” apart from the coding. They are completely interwoven in every game I’ve read. Every line, every paragraph, almost always has some amount of code within it, and a code structure that it fits within to have it flow from previous choices and scenarios. If the player has flirted with the character who’s speaking, or told them to kick rocks, or if the character’s father is dead due to the player’s choices, or if the character themself is dead, those all affect the dialogue the player has with the character, and each of those scenarios need some code and some different text.
I’m glad you’re enthusiastic! Let that enthusiasm fuel you over the coming months.