I find when I am writing one of three things tend to happen.
First, and best, is a productive session where I write lots and make good progress in the plot.
Second is that I stare at the screen for hours but type nothing. This happens more often then I would like.
Third, and worst of the set, is when I write for hours and then on reflection decide everything I have written is rubbish and delete it all. That is the kind of day I am having.