As a yoga teacher I am often asked the best way to get more flexible. I always tell people the same thing: you will get better results by stretching for fifteen minutes every day than by going to one yoga class a week. It is just the way the body works. Tiny bits of daily effort can add up to big results.
The same is true with your writing. If the only time you have to write are fifteen minutes squeezed in each day during your lunch break, then write your heart out for those fifteen minutes. Over a year you will get far more words written than someone who insists on having two full, undisturbed hours which they can only carve out every few weeks. (I say this with absolute authority because I have tried both ways.)
So, if you feel you are not getting enough time to write, try finding fifteen minutes in your day - every day - and write in that time. You might surprise yourself with how much you get accomplished. You may even write your book that way. There is a children's book author who got her start writing on her lunch hour. Her name is J. K. Rowling. As I said: tiny bits of daily efforts can add up to big results!