If you tell yourself you can only work in certain conditions (e.g. between 9 and 11 on partly cloudy days with 2.5 chocolate chunk cookies on a green napkin to the left of your computer keyboard), you will not get anything done! What I learned, very quickly, about working whenever the spare moment presents itself is that magical things can happen when you least expect it. You can be dog-tired and in no mood to write and yet, in a half-hour of messing around, you suddenly find your way through a tricky scene.
Snatch whatever minutes you can; be open to inspiration whenever, and wherever, it may occur. Sentence by sentence, the novel will get done.