You realise that the ironing board cover needs replacing.
This is the cover I made from a brand new cotton doona cover I bought from the op shop. It was the first time I had made one and I was very pleased with the result. You can read about it here.
I could never have told you how long an ironing board cover lasted but thanks to this blog I now know that this one has given 2.5 years of service. I regard that as pretty reaonable as I do a fair bit of ironing.