I think a common rule of thumb might be doing an 'annual' after 100 hrs/100 dives have elapsed......however, and DA Aquamaster can better answer this, I am assuming the O-rings and other soft parts in the annual kits are made of 'modern' (durable) materials as I would think such soft parts were originally made of older/obsolete/less durable materials and would require more frequent servicing 'back then