How long should you use a razor blade before changing it?

I have a Venus razor (can't remember which kind b/c I got it a long time ago). I've been shaving my legs and armpits for years now, but I never really know when I should change the razor blade. I think I may be using it too long before changing it. So how do you know when to change the razor blade?|||you should change it at least once a year because the blade will get dull easily. so change the blade often for a better shave|||when it doesn't shave well when it starts to no work as it used to at the start