You will understand if it is time to replace them or not by just looking at them. Like everything, they also rack up wear and tear. More than that, though, they also collect dead cells and oil from your hair and skin. This starts showing on themself. Another telltale sign that it is time for you to replace them is when they get all lumpy and saggy. They also tend to lose their fill and thickness as they age. So, you can just test for yourself if your pillow still feels comfortable or not.