The legal term quamdiu se bene gesserit means "as long as he shall behave himself well" (1) and is used when granting an office for an indeterminate time. See this entry in Black's Law Dictionary.

(1) Or she, I might add, not least in the light of the fact that in the novel Dune, the "Bene Gesserit" are an exclusively female order.

My question is simple: why is gesserit in the future perfect tense?

I also noticed this is not unheard of in classical Latin either; for example, a common phrase is quamdiu vixero (as long as I shall live), e.g. Socer‚ hoc enim te appellabo, quamdiu vixero (Sen. Controversiae 2,2); this also led to a somewhat inconclusive discussion in this previous question.

But anyway, I can find no justification for the future perfect in connection with quamdiu. What gives?