How often should brushes be replaced?

It depends on the usage patterns and type of brushes. Most makeup artists who use animal hair brushes end up replacing them every year or so. But some brushes are designed to last many years with normal use. If you suspect your brush has come to the end of its life and is no longer performing well for you, then retire it and just buy another. Artis Brushes are made using CosmeFibre which normally has a long usage life.