In my rule book, I don't have to do anything except sit at the computer, but I'm not allowed to do anything else, and I usually get so bored I start to work.
I think this strategy puts Bender in something of a minority. I'm used to hearing about word counts, like Stephen King's 3,000 per day (though he suggests 1,000 for mortals like us). I have used this myself. The idea is that you can't get up till you do your 1,000, so you might as well bang through it. Who knows, you could be done in half an hour, and off to more rewarding things like staring into the fridge and muttering "fuck."
But I'm interested in this time business. First, it accommodates those of us--most of us--with other jobs and/or obligations. If you "write" from 7-9 every morning, or 7-9 every evening, you can state with confidence when you will and will not be participating in the flesh-and-blood world. There is also, let's face it, something particularly draconian about the word count. On days when the words aren't coming, and you just can't bear to kick out your internal editor and pour out crap like you're supposed to, "597 to go" is a deeply lousy feeling. The time thing seems a little more gentle. Better to write from boredom than from obligation, perhaps.
Setting time frame seems especially applicable during revisions, when a word count doesn't really make sense.