2-3 months for us.
Cold setup. Superlite flocked dual trim. Mostly residential, 5 days a week.
Don't like it when they start to curl at the edges too much especially on the ends of the stock. Can't get the corners of frames as easy, especially on sash windows. The whole brush then starts to have a banana look to it.
Love getting a new brush, it's like having a new tooth brush. Just much more expensive!