It’s not a matter of how much time you spend on your art, it’s all about quality time. I often find that I do my best work in short bursts.
Staying focussed is so important when creating artworks. There is no problem in changing your mind about the finer details of your work, but you need to stick to your original plans as much as possible. Failure to do so may result in the work losing its appeal/aim and end up being consigned to a drawer or even the rubbish bin.