Very soon then the fëa and hroa of a Man in Aman would not be united and at peace, but would be opposed, to the great pain of both. The hroa being in full vigour and joy of life would cling to the fëa, lest its departure should bring death; and against death it would revolt as would a great beast in full life either flee from the hunter or turn savagely upon him. But the fëa would be as it were in prison, becoming ever more weary of all the delights of the hroa, until they were loathsome to it, longing ever more and more to be gone, until even those matters for its thought that it received through the hroa and its senses became meaningless. The Man would not be blessed, but accursed; and he would curse the Valar and Aman and all the things of Arda. And he would not willingly leave Aman, for that would mean rapid death, and he would have to be thrust forth with violence. But if he remained in Aman, what should he come to, ere Arda were at last fulfilled and he found release? Either his fëa would be wholly dominated by the hroa, and he would become more like a beast, though one tormented within. Or else, if his fëa were strong, it would leave the hroa. Then one of two things would happen: either this would be accomplished only in hate, by violence, and the hroa, in full life, would be rent and die in sudden agony; or else the fëa would in loathing and without pity desert the hroa, and it would live on, a witless body, not even a beast but a monster, a very work of Melkor in the midst of Aman, which the Valar themselves would fain destroy.

M.A.S.H. Trivia #11

But as for the Valar themselves, and the Maiar also in their degree: they could live at any speed of thought or motion which they chose or desired.* * They could move backward or forward in thought, and return again so swiftly that to those who were in their presence they did not appear to have moved. All that was past they could fully perceive; but being now in Time the future they could only perceive or explore in so far as its design was made clear to them in the Music, or as each one of them was specially concerned with this or that part of Eru's design, being His agent or Subcreator. In this way of perception they could foresee none of the acts of the Children, Elves and Men, in whose conceiving and introduction into Ea none of the Valar had played any part at all; concerning the Children they could only deduce likelihood, in the same way as can the Children themselves, though from a far greater knowledge of facts and the contributory events of the past, and with far greater intelligence and wisdom. Yet there always remained an uncertainty with regard to the words and deeds of Elves and Men in Time not yet unfolded.

On earth the Quendi suffered no sickness, and the health of their bodies was supported by the might of the longeval fëar. But their bodies, being of the stuff of Arda, were nonetheless not so enduring as their spirits; for the longevity of the Quendi was derived primarily from their fëar, whose nature or 'doom' was to abide in Arda until its end. Therefore, after the vitality of the hroa was expended in achieving full growth, it began to weaken or grow weary. Very slowly indeed, but to all the Quendi perceptibly. For a while it would be fortified and maintained by its indwelling fëa, and then its vitality would begin to ebb, and its desire for physical life and joy in it would pass ever more swiftly away. Then an Elf would begin (as they say now, for these things did not fully appear in the Elder Days) to 'fade', until the fëa as it were consumed the hroa until it remained only in the love and memory of the spirit that had inhabited it.

Let us suppose then that the Valar had also admitted to Aman some of the Atani, and (so that we may consider a whole life of a Man in such a state) that 'mortal' children were there born, as were children of the Eldar. Then, even though in Aman, a mortal child would still grow to maturity in some twenty years of the Sun, and the natural span of its life, the period of the cohesion of hroa and fëa, would be no more than, say, 100 years. Not much more, even though his body would suffer no sickness or disorder in Aman, where no such evils existed.