I have always been of the belief to replace it after 4-5 years of wear, depending on how you look after it. I never hang it off my mirrors as the edges will cause squashing of the foam inside. Always wash the material insides and wipe out to remove any abrasives that may get in there ie; dust, dirt, sand....
I guess the big thing is.... Do you feel safe with it? it's your head, if you want to replace your helmet every month because you feel safe doing so then go for it, if you want to wait a couple of years, do so. I wouldn't; however; wear a helmet for any more than 5 years as the oils in your hair will degrade the performance of it structurally.