Procastination is not the answer. It is theretically cheaper to pull them every six months and recoat them. Your type af driving milage accumulation etc might make a difference in the frequency a little. If they come out extremely easy at the six month point you might extend it a little for example.
One thing for sure it is far cheaper even perhaps if you have to pay an individual to do it over the long haul. The peace of mind is not insignifigant either. We all know some in not all glow plugs will fail eventually. I agree if already installed for a long period to wait and see.