good symbol for "hold note until breath runs out"?

So I'm writing a graphic score for voices and I need a notation for this. Any ideas? There is no rhythmic notation in the score, there's just a note with a line after it showing approximately how long it is to be held. I was thinking of having the end of the line sort of fade away to represent this (with the key in the performance notes of course) but I'm not totally sure....

I'd just notate it as a semi-breve, and insert text explaining what you'd like. I'd perhaps even notate a tie from the semi-breve as if it's tied to a following note.

Some random ideas:

The square fermata is typically used to designate an extra long fermata.

The infinity sign might indicate "as long as possible".

That's sort of like your "fade out" suggestion, just with the well-known meaning of an ellipsis as "continue like this". It also has the advantage that it works in black and white and doesn't rely on greyscale like your fade-out option, which might become less clear in some cases (such when your score is copied).

In all cases, definitely make a clear note in your performance notes, or directly in the score. I'd write it in the performance notes if it appears several times, or directly in the score if it appears a single time. Or maybe even write it in the performance notes, and a short "one breath" or similar above the note. Never hurts to be extra precise.

Hey thanks, I think I'll go with that last one actually:)