Hotkey wrote:Suzu route has an approximate total of 162,187 words.
Rika route has an approximate total of 44,362 words.
(All these numbers may be inaccurate).
So, consider how much you're comfortable writing per month. 10,000 words? That's 16 months for a Suzu-length route. 5,000 words? That's 9 months for a Rika-length route. As ProfAllister and Mournful3ch0 mentioned, you can do more but you'll suffer burnout and thus a likely reduction in story quality.
As listed in the archive:
Can You Open Your Heart? – Route by Rikabro (completed, 63,615 words)
A pseudo-pseudo Suzu route – Route by Scissorlips (completed, over 240.000 words)
If you want some really scary numbers, over the ~18 months I've worked on Tomorrow's Doom (I'm approaching 19 months since I started), I've written ~25,000 words per month (450,000 total, unfinished). That's an average, mind you. The first 15 chapters were written and released between July 1 and August 4th 2012, and totaled ~115,000 words. That's around 3200 words per day. I'm still working on it, almost daily. I wrote 2200 words last night.
It. Is. Difficult.
There have been times I've gone for almost a month without writing anything meaningful. More than once, almost weekly, I consider giving it up and throwing in the towel. Even as I've been using almost an entire cast of original characters, it's hard to keep them straight, make sure they come through, and continue impacting the story.
My chapters are all neatly separated into their own files, but along with them are about 40 additional scrap files: chapters started and stopped, story threads that didn't quite make the cut, outline notations and dramatis personae. There's probably another 100,000 words of junk files collecting digital dust in a raggedy folder - probably more, actually.
Just to distract myself over the months, I've written other things as well. This past Christmas I put a week and then some into a holiday story about Hanako and Taro that remains unfinished, but probably spans ~32,000 words. Then there's The Kenji Files, a crossover fic with Kenji and Emi as secret agents that I've put about 20,000 words into - 5,000 or so unreleased.
Just be careful about what you're getting yourself into. If I hadn't had a clear idea of the story I wanted to tell all those months ago, I never would have gotten as far as I have. The details weren't all aligned, and I've muddled some things along the way, but I started knowing the character I was creating, her friends, their relationships, how they think, act, where they're from, what they want, and that for a main cast of 6 people, plus another 4 that got extra development along the way. I'm committed at this point, but after over 400,000 words, I'd better finish.
Think about that before you leap into making a full-length book.