Good quality threads manufactured these days will last much longer than threads produced 20 years ago. We are lucky to use today’s best quality cotton threads because of the advanced processing techniques used to make new cotton threads. The high-quality threads produced today may still be fine to use 40 to 50 years from now. High-quality modern threads will last longer because of the improved process of making threads these days. The development of genetic engineering also helped in producing better cotton threads. There is a simple way to test if the cotton threads you have bought or the old stash at home is still good enough to use in your machine. Hold a foot of the thread between both your hands and then pull it apart. If the thread snaps with a nice, crisp break, then the thread is still fine to use. But, if the thread separates and easily pulls apart, it is no good to use. The polyester thread may fade through time with exposure to sunlight. But old polyester thread does not deteriorate as cotton threads do. This observation confirms that synthetic threads are more durable. But for you to have a successful sewing project, it is still best to use new threads of the best quality you can afford.