Writing for extended periods of time will gradually decrease the quality of your writing. Or, it does for me. I suggest writing for 3 hours tops, but at a point I find my writing starts to get sloppy. That will only make proofreading your book harder when you come across your "sloppy" areas. I notice when my writing starts to lack in quality, and I take at least an hour break before coming back. Writing is fun, but writing for too long is something I wouldn't suggest. Especially if you are writing in the perspective of a villain, it is said that you absorb some of the villain's qualities and moods when writing in his/her perspective. So if you are writing in the villain's perspective, be careful on how long you write. All in all, MY opinion of writing for too long is as follows: Take plenty of breaks and don't continue writing for an extended amount of time.