Snooping software can’t exactly tell how long you spend on a site. You can infer it I suppose by the first time in a day that you hit the URL and the last time, but there’s no telling how much you were multi-tasking, etc. For example, you could keep Scottrade up in a window just to keep tabs on things. Stupid ass Farmville, I was hooked on that about a year ago.
You can't get an exact time, but you can get pretty darned close. The software looks at how often the page is refreshed, links clicked, etc. I do this for a living, and I have had people try to argue your point when caught wasting time. They always lose...