If the tip is bent way out of shape or if the tip is rusty, you will need a new tip.
If you find that the tip isn't conducting heat as well as it used to you need a new tip.
Obviously if the tip isn't the shape you want it to be you need a new tip.
If you want your tip to last longer, clean it regularily by melting solder over it and then wipe it off on the wet sponge.
Also every time you put it away "tin" the tip (melt a light coat of solder over the tip).