How to Say "only as long as" in Latin?

Question

In 1598 French King, Henri Quatre, passed the "Edict of Nantes", to protect French Huguenots from persecution by members of the Catholic majority. Almost a century later, his grandson, Louis XIV, repealed this legislation (1685).

Describing these events, in his TV-prog., "The Christians" (1977), the late Bamber Gascoigne said:

"But this help from the king (Henri IV) could last only as long as the king."

In translating this into Latin the tricky bit would appear to be, "only as long as"; is it simply a double-adverb, "solum quamdiu"? Simplistic, word-for-word translations are hardly ever correct. Anyway, here's a guess:

"hoc auxilium rege solum quamdiu rex permanere posset."

Is this correct?

Answer 1

Your approach isn't entirely wrong (well, if we gloss over the non-existing form rego); however,

1. quamdiu stands with the indicative
2. I believe that sharing a verb between the main clause and the subordinate clause introduced by quamdiu is at least extremely unidiomatic, so you have to repeat permanere or use perhaps a different verb for the king.

So for example:

Hoc auxilium regis solum permanere potuit, quamdiu rex vivebat.

Instead of quamdiu, you could also say quoad or dum. But I'm not sure that solum quamdiu is classical. I would perhaps have written:

Hoc patrocinium regis tantummodo permanere potuit, dum rex vivebat.

If you want to keep the construction of the help and the king both “lasting” (without repeating permanere), then you could avoid a subordinate clause and instead write:

Hoc auxilium solum tamdiu quam rex permanere potuit.

Like for solum quamdiu, there are lots and lots of Neo-Latin Google hits for solum tamdiu, but again, I'm not sure about Classical Latin.

However, what would be more elegant in my opinion would be the approach suggested by Kingshorsey in his comment, to wit, using negation with a comparative:

Hoc auxilium regis diutius quam rex permanere nequiit.

Answer 2

Hoc auxilium regis tantum permanere poterat dummodo rex ipse in regno [imperio, potestate] permaneret.

There’s an implied condition in this “as long as” construction: the help lasts only if the king lasts. That’s why I favored dummodo plus subjunctive.

Compare Ars Amatoria 2.275-276, where Ovid observes that girls care more about money than poetry:

Carmina laudantur, sed munera magna petuntur:
    dummodo sit dives, barbarus ipse placet.

And Metamorphoses 9.29-30, Hercules’ scornful dismissal of Achelous’ boasting:

                 Melior mihi dextera linguā.
Dummodo pugnando superem, tu vince loquendo.