How often do you wash your towels? And how often do you completely change them with brand new towels?

I wash them weekly and was just wondering recently if I should be buying new towels to replace my 8+ year old towels.

Edit to add: I wash towels by themselves so it’s a separate load to my regular laundry. I don’t know why but the different materials seem to clean better separately. But I also have a separate towels load for dirty/oily towels (kitchen towels, cleaning rags)