When does the axle seal need to be replaced in a car?(00)

When does the axle seal need to be replaced in a car?(00)

Image gallery: Axle Gasket Leak

When does the axle seal need to be replaced in a car? Is it Safe to Drive with a Leaking Axle Seal? YourMechanic Advice How To Fix A Leaking Rear Axle (Replace Axle Seals)