Ya.. it's all relative.

Back in my EQ days, my "casual" guild would raid 6 hours a night, four nights a week.
Some of my friends were in "hard core" guilds who raided 8 hours a night, six nights a week.

These days, I'd probably consider most wow guilds that put in 24 hrs a week towards progression to be hard core.
Maybe not necessarily a top 5 guild across all servers, but definitely able to defeat all content fairly quickly.
My current guild raids 4 hrs on Tue/Thu and 2 hrs on Sun, and since the start of BC has basically seen everything.