The Zen folk have a saying: you should meditate for twenty minutes every day, except on days when you don’t have the time. In which case you should meditate for an hour.
I think the same principle applies to writing – write for four hours a day, except when you don’t have the time. In which case, you’ll need to write for eight hours. And you still won’t catch up with yourself.
Photo: 'Zen' by Louise Leclerc via Flickr and Creative Commons