I just took apart a bench seat cover from a 1965 pickup truck, and the thread, which I assume was nylon, was still strong. Nylon (except monofilament) and polyester will be fine for a long, long time. Only thread that has cotton content, or some other natural fiber needs to be worried about.
I recommend that people who have thread given to them or it came with a sewing machine throw it away unless you know for sure the fiber content, if it is left or right twist, and if it is bonded thread. Bonded thread has a lubricant built into it so it passes through the machine easier. Industrial sewing machines need to use left twist thread. Right twist thread will unravel because of the direction the hook travels, which is to the right, or clockwise.
You can figure out which twist the thread has by holding a piece of thread about 6 inches long firmly between the thumb and forefinger of your left hand and then rolling the other end of the thread toward you between the thumb and forefinger of the right hand. If it tightens, it is left twist thread. If it loosens, it is right twist thread.
The picture shows the labels on the bottom of both of these spools of thread. I was given the orange thread. It is B-138, which is its size, but the "B" also tells me it is bonded, even though it says so on the label. It is also nylon, but it doesn't tell me what the twist is. Luckily it is left twist, so I will be able to use it someday. The other one is B-69, which means it is half the diameter of the orange thread, it is bonded, and it is nylon. Unfortunately, the labels sometimes come off long before the thread is used up. When I get a spool like that, I throw it away.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 02-21-2010 at 02:34 PM.