Supposedly he's had the ending planned out for ages. He just doesn't seem to know how to get there. It's also been 11 years since Dance came out. It's not the show's fault he's taking so long.

Of course he can take as long as he wants. Hell, he could retire right now and leave the books unfinished if he wanted to. But that doesn't mean people can't still be frustrated that he's not finishing what he's started.