It usually takes 12 months, from start to finish, to write a novel. This includes all planning, plotting, writing, rewriting, more rewriting, editing and then a final rewrite.
I have lots of ideas for books that I write down in my little black book, or perhaps flesh out with a page of A4. But I tend to focus just on one project at a time, get that written and then move on to the next thing, otherwise I find that I never finish anything. However, sometimes you need to have a break from a manuscript, at which point I'll park the book, go and write something else, usually completely different, and return to it when I've had a chance to clear my head and look at it more objectively.
Every stage has its little pleasures. Finishing a book is, of course, a great moment, but such is the nature of writing books you are never entirely sure you are done.
When you're deeply immersed in a book and the words are flowing, that's a feeling unlike any other. Every writer lives for that moment.
But I think my favourite part is when you first strike out with a book and begin writing it. It's that moment of excitement, wondering where it will lead you and what it'll end up like when you're done. It's never as you first envisage.