However, to keep matters simple, let us stay here with modern, good quality Dacron. In other words, the white sails most commonly seen on cruising boats and yachts of average size.
The cloth itself will probably hold together very long, before it actually physically tears or rips apart due to old age and excessive exposure to UV rays. But long before this, the sail itself will have “blown itself out”, meaning that it will have long lost its original shape. And this is much less easy to detect for the average cruising person, than actual damage to the sail.
If sails are used on a daily basis in the tropics, and not stowed away under cover out of the sun when not in use, the life-span can also be much shorter than the average, due to excessive UV-influence, as can be seen on many charter boats in the Caribbean or Pacific.