I put about five hundred hours or so into DKS and can't play it anymore. I know some people go longer, but I can't.
I just recently got Demon Souls and finished my first playthrough in 17 hours. Overall it was pretty easy coming from DKS. I will probably play one more time through, but already there isn't much more I want to do in it. I feel like by now I got the Souls formula down. As long as you tread cautiously you will always prevail. So now I am wondering about the next game, and whether or not I am going to care about playing it.
I think now, after having got another Soul fix from DES, I am just about burnt out on Souls games, and now I am mostly looking forward to whatever Miyazaki creates next. I would definitely enjoy a challenging game set in a more "happy" environment. I get tired of the constant dark and depressing games. I am currently working on Okami HD, and while it is nowhere near as fun as DKS or DES, the light refreshing atmosphere is like a breath of fresh air in a dank porto-sh1tter.