There are times when I really feel like writing. There are times when I feel like writing and feel particularly creative - those times are awesome!
There are many times, however, when i don't feel all that creative nor feel too inspired to write. This doesn't appear to be something unique to me, which is nice to know that at least I'm not alone.
But what should you do when you feel this way? I can't speak for everyone, but my answer is to keep writing. Maybe you don't write a lot. Maybe you don't write anything that is particularly good. But you keep the process going. Writing becomes a habit.
The thing to remember is that you are going to edit your work before it is released to anyone to read. So even if you end up writing some poor stuff, that is okay - you'll fix it later.
The important thing to do is keep going. One regret I have was when I stopped jogging. It took awhile to get into a groove, but once I was there, I actually enjoyed it. After a running event (which was the reason I started jogging) I took some days off. Then I took some more. Before long, running was a chore again. If only I didn't take those days off!
Writing is not exactly the same, but I firmly believe that consistency will help you to keep going. Creating a novel is not an easy thing and takes time. Don't make it even harder on yourself by taking long breaks from writing.
Remember, good or bad, taking time to write is an accomplishment!