Elves only have children once in their life, and it saps a lot of strength so they don't do it during war.
In circa 1959, J.R.R. Tolkien wrote a series of texts which he collectively called "Of Time in Arda". These are published in The Nature of Middle-earth.
It may also be noted that in each Elvish life there was normally only one period of begetting or bearing children, whenever begun; and that the length of this period was variable, as were the number of children produced. It might occupy from 12 to about 60 years (occasionally more). The children numbered usually 2, 3, or 4. ...
Thus we may observe that all matters of growth and development, which belong to the separate nature of the hröa, engaged in its own process of achieving its complete and mature form, and which are not under the will or conscious control of the fëa, proceed far slower among the Quendi than among Men. Gestation, therefore, proceeds according to the growth and ageing scale of the Quendi, and occupies ¾ yên, or 108 MY[= Middle-earth years].
During all this time the parents are aware of the growth of the unborn child, and live in much longer and more deeply-felt joy and expectation; for childbirth is not among the Eldar accompanied by pain. It is nonetheless not an easy or light matter, for it is achieved by a much greater expense of the vigour of hröa and fëa (of “youth” as the Eldar say) than is usual among Men; and is followed after the begetting by a time of quiescence and withdrawal. The Elf-women also are usually quiescent and withdrawn before and after birth. For these reasons, the Eldar did not (if they could avoid it) enter into the “Time of the Children” in times of trouble, or wandering. There were thus no marriages or births during the Great March; nor again during the journey of the Ñoldor from Aman to Beleriand, and births were few during all the War against Morgoth. For the same cause, Men who had dealings with the Eldar often saw far less of the Elf-women, and might even be unaware that some Elven-king or lord had a wife. For the withdrawal and quiescence of the wife might occupy the whole time of his sojourn among the Eldar, or indeed much of his whole mortal life-time. For this “withdrawal”, occupying from three to four “months” or twelfths of a “year”, that is one quarter to one third of a yên, would in mortal terms endure for about 36 to 48 years.
The Nature of Middle-earth - "The Quendi compared with Men"
In the case of Elf-women: marriage and child-bearing took place earlier, their first child being born before they were of age 20. Later indeed some postponement was usual so that marriage at 21 was the most usual time; though any age up to 36 (18 + 18) was not uncommon. In days of trouble, or of travel and unsettled life, the begetting of children was naturally avoided or postponed; and since the postponement especially of the first child-bearing or begetting prolonged the “youth” or physical vigour of the Quendi, this might occur up to a female age of about 72 (18 + 54) – but a first child-bearing seldom occurred after this age. ...
At all times, unless circumstances interfered and separation were forced upon spouses by wars or exile, the Quendi desired to dwell in company with husband or wife during the bearing of a child and its early growth. Also as a rule, they preferred to arrange their lives so as to have a consecutive “Time of the Children” in which all of their children were born – but this of course often, especially in the troubled early years, proved impossible. After a birth, even if a consecutive Onnalúmë or ‘Time of Children’ was achieved, a rest was naturally always taken. This was governed by “growth-time”, and so was usually not less than 12 löar (= 1 growth-year); but it might be much more; and usually was increased progressively between each birth for consecutive series: as 12 : 24 : 36 : 48 etc.
The Nature of Middle-earth - "Natural Youth and Growth of the Quendi"
A few points may be noted:
- Elves like to arrange all of their childbearing so that it happens in a continuous interval
- Depending on the number of children this interval can be anywhere from 144 to 720 years. (They need progressively larger recovery periods between each child.
- They like to plan their lives so that this one period happens during a time of peace, when both parents can be home