There are a lot of factors that go into how quickly you will need to replace your gloves so there is no general rule of thumb. Good quality gloves that are being used every day in all conditions may need to be replaced more than once a year however cheaper gloves that are only used in summer will last a lot longer. If you notice any serious fading or rubbing and feel that it is impacting the gloves protection level then it is probably time to start thinking about some new gloves.