My first manuscript took me two and a half years. My second took me just over 6 months.
Before writing the second, I had written the basic idea as a prologue chapter within the first book. The writing then became, in essence, growing the 'seed' chapter into a novel.
One other thing that helped me plan and execute my writing was to sort out an easy-to-use organising system.
I've always found mind-mapping useful in terms of organising and developing ideas and seeing the connections between ideas. I recommend iThoughts if you use Apple products. Scrivener has Scapple, but I've not been impressed with it when I tried the trial.
Although some of these mind-mapping apps can be great, they can take time to learn and become proficient at. I wanted something more immediate and tactile. One day I put up a magnetic whiteboard on an 'out of the way' wall (so that others don't have to look at it every day).
Now I plot my book by writing short descriptions of scenes onto post-its and placing small magnets on each one. The post-its are colour-coded by content or character involved. The post-it's are then arranged in three lines - Act 1, Act 2 and Act3, which each line headed by a basic description of that Act. I have blue magnets to signify where the plot points occur. When I've written a scene, I usually affix a small sticker to the relevant post-it note. I use white post-its as a kind of 'place holder' scene - when I'm sketching out a framework, but am not exactly sure about the perspective or content. Eventually the white post-its will be replaced by coloured ones as I sure up my ideas.
The photo shows my set-up for book 3 (the 3 lines at the top of the board). I get immediate visual feedback on the structure of my work. I can see that Act 1 is about 25% of the book and Act 2 is twice that. Getting that right helps me with general structuring and pacing. Below the horizontal bar is my initial planning and set-up for book 4. To the right is a set of post-its that have disconnected scene ideas that relate to either the book series as a whole or is stuff for later books.
It's easy to shift scenes around this way and you can add stickers for things like foreshadowing and reveals or make other types of connections using markers.
I don't write in a linear way - sometimes I wake up and have a new insight about something or have a particular scene fill my head. Planning in this way means I can write scenes and make the connections between in a more organic manner.
Hope this helps someone!