Indication of Product Build-Up

Happy Sunday everyone!
So, last week I co-washed my hair last week. Two days later, my hair became sooo dry.
I tried moisturizing it, but it remained dry. I realized that my hair needed a good wash because
of all of the product build up. I do not know how some people only do co-washes lol. I guess it depends
on your hair and the products that you use.