Yeah, I would imagine that each QC comic takes a solid 4 hours (from start to finish). Jeph probably has found some shortcuts (recycling the occassional background, etc) but, unlike many webcomics, QC is consistently dynamic. I don't think I could go back through the archives and find a single frame that has been copied, and pasted into another comic down the road.
As for getting a new comic out daily, it really wouldn't be that difficult (from a technical standpoint). Consider that this comic is Jeph's full-time job... so it's not like he's spending 40 hours in an office, then spending another 4 doing the comic, and an additional few more hours dealing with merchandising. THAT would be insane!
As for Jeph keeping the story fresh, daily, and constantly writing humorous/interesting dialogue... that's just a gift, plain and simple.