A generic or universal statement cannot be made. It’s related to the factors of regenerant chemicals consumption,  the water quality and the cost of waste stream disposal/limitation. Some ion exchange plant managers would consider a capacity loss of 10-20% sufficiently high to justify the total resin replacement but all plants are different in terms of the factors of the water quality, costs of regenerant chemicals and waste stream disposal. The price of regenerant chemicals plays a very important role since the cost per volume of product water produced increases when more chemicals are needed to regenerate old resins. With increasing environmental concerns everywhere, waste stream disposal is now getting more attention and restrictions to the operators. As a rule of thumb, it is more economical to replace the anion resins.