Options I see:
- You could use Until, but "until the fire dies out" would be a poor choice, since it would be a ritual;
- An alternative could be using Bargain, duration sun/moon, and setting a condition like keeping the candle lit up, but that also seems like a poor choice, magnitude-wise.
- Inner mysteries offer a few alternatives, but most are unsatisfactory. Perhaps the one duration that might be handy here is the While (condition). The condition requires an activity from the target, so you might end up with something like "While holding a lit candle" which might not be handy if the target is expected to dance, etc. "While attending a social event" could be an alternative that would make sense, magnitude-wise.