Por quanto tempo a escrita é visível depende de quão quente foi o Ring
Esta resposta é baseada no livro e não nos filmes. Somos informados de duas vezes quando a escrita era visível.
Pergaminho de Isildur
Gandalf relata ao Conselho de Elrond que ele leu um pergaminho feito por Isildur em Minas Anor (conhecido posteriormente como Minas Tirith).
‘For Isildur did not march away straight from the war in Mordor, as some have told the tale.’
‘Some in the North, maybe,’ Boromir broke in. ‘All know in Gondor that he went first to Minas Anor and dwelt a while with his nephew Meneldil, instructing him, before he committed to him the rule of the South Kingdom. In that time he planted there the last sapling of the White Tree in memory of his brother.’
‘But in that time also he made this scroll,’ said Gandalf;
The Lord of the Rings, Book II, Chapter 2: The Council of Elrond
No pergaminho, Isildur afirma que o anel estava muito quente quando Isildur o pegou do dedo de Sauron (quente o suficiente para queimar), e a escrita não havia desaparecido completamente quando ele estava fazendo o pergaminho. Como Daniel aponta em sua resposta, Minas Anor está a vários dias de jornada da batalha.
It was hot when I first took it, hot as a glede, and my hand was scorched, so that I doubt if ever again I shall be free of the pain of it. Yet even as I write it is cooled, and it seemeth to shrink, though it loseth neither its beauty nor its shape. Already the writing upon it, which at first was as clear as red flame, fadeth and is now only barely to be read.
The Lord of the Rings, Book I, Chapter 2: The Council of Elrond
Experimento de Gandalf
Em A Sombra do Passado , Gandalf visita Frodo em Bag End e quer testar sua strong suspeita de que o anel de Frodo é o Um Anel. Ele joga o anel no fogo. Quando, depois de "um tempo", ele tira o anel do fogo, é legal, mas a escrita é visível.
To Frodo’s astonishment and distress the wizard threw it suddenly into the middle of a glowing corner of the fire. Frodo gave a cry and groped for the tongs; but Gandalf held him back.
‘Wait!’ he said in a commanding voice, giving Frodo a quick look from under his bristling brows.
No apparent change came over the ring. After a while Gandalf got up, closed the shutters outside the window, and
drew the curtains. The room became dark and silent, though the clack of Sam’s shears, now nearer to the windows, could
still be heard faintly from the garden. For a moment the wizard stood looking at the fire; then he stooped and removed
the ring to the hearth with the tongs, and at once picked it up. Frodo gasped.
‘It is quite cool,’ said Gandalf. ‘Take it!’ Frodo received it on his shrinking palm: it seemed to have become thicker and
heavier than ever.
‘Hold it up!’ said Gandalf. ‘And look closely!’
As Frodo did so, he now saw fine lines, finer than the finest pen-strokes, running along the ring, outside and inside: lines of fire that seemed to form the letters of a flowing script. They shone piercingly bright, and yet remote, as if out of a great depth.
The Lord of the Rings, Book II, Chapter 2: The Shadow of the Past
Segue-se uma conversa que pode ter levado 20 ou 30 minutos antes que Frodo olhe novamente para o Anel e a escrita tenha desaparecido.
Frodo drew the Ring out of his pocket again and looked at it. It now appeared plain and smooth, without mark or device that he could see.
The Lord of the Rings, Book II, Chapter 2: The Shadow of the Past
Conclusão
A partir dessas duas evidências, parece razoável supor que quanto mais quente o anel se torna, mais longa a escrita, com o intervalo sendo pelo menos de vários minutos a vários dias.
Isso faz sentido intuitivo. O metal vai brilhar se você aquecê-lo o suficiente, e vai brilhar mais se você torná-lo mais quente. Este caso não é exatamente o mesmo que não é o metal, mas a escrita que está brilhando, e no caso do experimento de Gandalf, o metal não é nada quente.