Keep A Regular Schedule
First of all, I think it's important for you to write AT LEAST once or twice a week. This is necessary not only to keep your writing skills sharp, but also for you to remember where exactly you are with your story and what your main characters are like. If you have too many weeks where you write nothing then there is the all-too-real danger that either you will never finish your book or else you will complete it only to realise that large chunks are not good enough and need significant revision.
Do I Need To Write Every Day?
Ultimately this depends on how much time you have and what your schedule is like. Personally speaking, I don't write every day, but I do write 4 or 5 times a week, typically producing 1000-2500 words a sitting, which equates to approximately one chapter a day. This ensures that not only do I keep in a writing frame of mind, but that I also push it towards completion. Ultimately, you need to ask yourself am I writing enough? Can I write more? If the answer is yes then you need to write more - it's as simple as that.
Make Sure You Finish Your Book
The object of writing is not merely to write every day or a few times a week. No, the aim is to produce quality work on a regular basis. Don't write for writing's sake or bounce off an extra hundred words just to make your word limit for the day. You might find it helpful to set yourself daily or even monthly targets - I do. Indeed, I frequently aim to finish the composition and editing of a book inside 9-12 months. Overall this strategy has helped me write 4 novels and one full collection of poetry in a little over three years. Not bad eh?
Don't forget to Make Time to Read
Writing isn't the only way to improve your author skill set - reading plays a big part too. Stephen King recommends that you read for 3-4 hours a day. Most people - myself included - would find that very challenging, so I recommend at least 1-2 hours of quality reading. This way you get to see not only what is currently getting published but also what language is used and what characters are in vogue etc, thus making you a better, more knowledgeable writer.
Don't Forget To Make Time To Promote
It's important for every author to promote their work, especially if it's been published by the mainstream or independent press. Try to find time to share your book with your friends on Facebook, on Twitter, and on Reddit etc. It's also a good idea - like I'm doing now - to write a blog and in so doing inform the reader about your book and any promotions that you might have. This way you will sell more books, get more reviews, and see your confidence levels shoot up in the process, thus helping you to write more in both the short and long term.
This weekend I'm running a giveaway for my Jack Strong series of books about a 12 year old boy who finds himself on an alien spaceship millions of light years from Earth. Both books - the links are below - are free all weekend!
Jack Strong and the Red Giant:
Jack Strong and the Prisoner of Haa'drath: