I use the wash 65 times rule, if it starts to look sad and weary, out it goes, or if, as sometimes happens, I never really love it. The only thing that niggles me on this is that sometimes, the fabric has been expensive.
At the moment, I think, the oldest thing I have from the early 80's is a pure silk jacket that I made from silk bought by my mum from Thailand/Hong Kong (can't remember which). She got miffed because I made a jacket instead of a dress, which saved it's life. Up and down in weight yet it still fitted me. Only thing is that since there are years between each wearing, it has never been washed. I kept the leftovers, I ought to try washing a piece.
I have also kept a skirt, lovely summery fabric, which at one time I must have used for painting (again early 80's). Every now and again I hold it up and admire the size of the waist