A well maintained car shouldn't ever need any part of the head replacing. As Ed says, occasionally they can get noisy, but it's not worth replacing them. My brother in law has an old Astra GSi and they were renowned for noisy tappets, I believe he's about to exceed 200k miles and only one or two have got noisy.
Just keep on top of the oil changes and use good oil. And don't forget the cambelt changes!
