Ideally this should be done before it breaks and leaves you stranded on the side of the road. Your CV axles are an important component in your car and when it's advised to replace them, then that's the right time. If the boot on your CV axle is cracked and not leaking then it can wait, but if the boot is torn and the grease is leaking out then that is the time to do it. Once the boot is torn or broken then water and dirt can get into the joint and cause internal damage.
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