Last week I stumbled upon a small, free, simple but ingenious application - Active Timer, written by a Robert Chin.
What does it do? You simply start it and put it in the background or minimize it. It then will automatically keep a log of all applications you work in (or more precisely, it counts the seconds they have focus, whether you do something with them or not). So, at the end of the day you know precisely how long you have been in which application. Plus you may export the log in CSV-format, so you can open it in any spreadsheet and make your calculations.
I did this for several days and I observe astonishing things. For example, when I have the feeling in the evening that wow, today I almost only wrote on my novel, just peeked here and there into the 'net, had a few minutes look in some forum or on the news… - then my writing time is in fact only at 37%. Or 42%. That means, half of the time I do something else. And the amount of time in Safari is huge.
Next step is clear: Change my habits. If I arrive at 80% writing time, I’ll have doubled my productivity.
But maybe you will not know it. Because reducing forum time is a topmost priority now…