名词做同位语时需加逗号么But now it is realized that supplies of some of

名词做同位语时需加逗号么
But now it is realized that supplies of some of them are limited,and it is even possible to give a reasonable estimate of their “expectation of life”,the time it will take to exhaust all known sources and reserves of these materials.
名词做同位语时前面需加逗号么 怎么有的句子没有加,而有的却加了逗号?如这个句子 the time 前面加了都好的