I probably am not a good one to ask how long to keep pillows, because in my opinion, it takes months to get them broke in! Getting the kind, with zipped, removable covers, will make them last longer. Also, when they get too stained and flat, I guess it is time to think about replacing them. Unless, of course, you like flat pillows.
The point of a pillow is just to support your head while sleeping. But, in fact, you don't even really need a pillow. Many people sleep without. So... as long as your pillow is giving you comfort and allowing you to sleep easily, then it shouldn't be a problem. If you're worried about germs or bacteria, there was already an invention to deal with this: pillow case.