I think most people tend to have limits with how long a book or series they are willing to read. I tend to crap out on a book after about 1200 pages, and even that is seriously pushing my limit. On a series, if it's longer than 10 books I have a hard time justifying starting it unless it's absolutely amazing. I tend to think that if the story isn't finished in 10 books, it's going on waaaay too long. Another quibble I have with series is if they are unfinished. I try very hard not to start an unfinished series, but I routinely fail at that.
So, how long is too long for a book and a series? Or are you one of those readers where length doesn't matter as long as the story is compelling?