I would say at least 6 to 8 hours a day minimum. Even more on those days when I am on the road a great deal.
I have now turned about 10 people onto Slacker and they all have subscription with either a G2 or using their iPod Touch or iPhone for the service.
Just for the record all these folks are former sat radio subscribers who have canceled! Sometimes I wonder why I don't cancel as I hardly even listen to sat radio anymore. I think I listened to some news on their this afternoon for first time in a couple of week. I think the only real reason I keep it because of Doctor Radio and the NFL. If the NFL does what MLB did by coming up with an application for the iPhone, I might just cancel my service totally!