The only cassette I've ever had that was irreplaceable was a mix tape my first boyfriend made for me. He's a fantastic DJ and it's actually mixed, not just one song after another. I couldn't possibly guess how many times I've listened to it. What I can say is when the last time was. 2004, as I was packing up my beloved apartment that I had to give up because I wasn't working and could no longer afford it. I had a Nickelodeon boombox that I'd gotten specifically to play it, and listened to it several times over while moving out of that place. Not sure if I kept that boombox much longer, but I don't remember using it after that.
I was looking for something recently and came across the tape in a box in my office closet. I figured it was time to finally get the whole thing back in my library, so I found a USB cassette player on Amazon for $16. It's recording now.
This is actually version 2 of this tape. The original one broke, so I taped it together and made a copy. There's a gap in the music somewhere that I'll recognize when I listen. Obviously the quality here's not going to be great, but it'll still be wonderful to hear it again.