Say you have one MMO account open. You log in at least once a day to check your auctions - or perhaps a few times a day, chatting with guildmates while you're there. In the evening, you come home and to relax, you group with some friends or just go exploring. If you log in for three hours a day - even when it's spread out here and there - you'll clock 21 hours in a week. To some, this may very well be hardcore - to others, it's spread out in bite-sized pieces, so it feels pretty casual. If you have more than one MMO account, the time multiplies just in terms of simple maintenance like auctions and mail as well as social time; depending on your level of investiture of course.
Now, with all that said, we figured we'd ask - how precisely do you define casual? Is it time invested? Mindset and how you play? A bit of both? Something else entirely?