How long do elements last for?
ten undecillion years. or 10 to the power of 36 years.
There is no such thing as a fully fiberglass roof. If there was, it would'nt last very long as the materials inovolved in fiberglass (i.e. mainly glass) are not intended to withstand direct exposure to the elements. If you are looking for how long fiberglass insulation in your roof will last, your best bet would be to speak to a houseing contractor.
It is best to balance free elements last simply because it is often easier this way. When balancing elements in compounds, the ratios of the elements must be taken into account. However, when free elements are balanced, there is no increase in any other element. Thus, free elements are balanced in the end to compensate for changes in the balance of compounds.