I've had a number of Lee molds for over 20 years and they still cast fine.

One thing I do though is never leave them hot and empty to cool. When I am done casting I leave the last projectile in the mold and let the mold cool with a projectile in the mold. That way the mold never twists, or shifts and I think it helps the mold cool more uniform. This was something an old time caster told me to do. Whether its necessary or not, I could not say, but it seems to work for me.