I think mine started conceptually 5+ years ago.
But in terms of writing, I've probably worked at it since around January. So in ten months I've written I think just over 100 game pages, which, I feel probably isn't so much as I could have done.
I was at Uni most the time until August, then moving house... Then I'm also a nut for gaming particularly RPGs, so my story occasionally found itself on the backburner as I played games, The Last of Us being the latest to distract me.
And of course, there's all the days I just cant be bothered, or I have no inspiration to write. Sometimes a good game will spark the creative juices and get me at the writing board again but it's rare.
Anyway, I haven't finished yet! I don't think you need to put a date on when to finish your story by and honestly, I don't think you need to worry about how long the game is taking you. What matters is it is written how you want it, without pressure or the need to feel rushed. Just enjoy it, it'll be out when it's done and not a moment sooner.