I just bought an embroidery sewing machine and here's my question.  How long does thread actually last before it starts to fray/break?  I'm wondering if I should invest in a nice set of threads of just buy the colors I need as I need them.  So far, I'm just using my sewing thread and it is doing fine.  I've started putting the date on the bottom of each spool of thead I buy, I think I'm going to put my thread on a four year rotation...but I'm not sure.  I think old thread is part of what killed my old machine (though it was just a cheap $50 black friday number). 

My understanding is that when thread goes bad it becomes brittle and breaks easily.  I haven't heard a time frame for this, I believe it depends on the relative humidity in your area.  I have thread that must be over 15 years old, it was my mother's, and it seems fine.  I've also read that you can prolong the life of thread by storing it in the refrigerator, but I think I would just buy it as I need it than go to the trouble of keeping it there.  I hope this helps.

You may be able to salvage questionable thread, stored under fluorescent lights or bad air conditions, ect. by unwinding off the top layer of thread.  The thread underneath may be fine. This is for large spools on plastic cones.  If the thread seems weak or frays a lot, it may actually be that you need to change your needles more often.  Also a needle installed even slightly crooked will fray the thread.