The reason they last so long is because I darn them. As soon as that little hole appears at the toe (it's always at the toe) I darn them with a little black thread, it takes a couple of minutes and the tights live for many a day more.
I couldn't throw them out just for the sake of a few stitches, it's such a waste of money. I now have about 18 pairs all in very good condition, except for the little darn here and there. I wear them once, then wash them by hand, they're difficult to wear out !
When they do eventually become too worn to save, I cut them into strips and use them for garden ties. A little darning goes a long way.
Happy Frugalling .....