[steve-b]

You do not need to replace after one half life, but a good rule of thumb is that after 5 half-lives the isotope needs replenishing.

That depends on the nature of the decay product. For instance, tritium needs to be replaced much sooner than a single half-life (12 yr), because its decay product (He3) soaks up neutrons like a sponge and muffles the chain reaction.

[NukeMan]

Correct.