Brushes service life
Brushes are a wearable item and should be budgeted to be changed every 120 - 150 hours of operational use. These figures will vary and be dependent on spindle RPM, material being cut and depth of cut in one pass.
Only use original spare parts when changing brushes, which can be found here.
Servicing of the spindle
Good practise is to regularly (every 15 to 20 working days dependent on use) remove the spindle from the Z Head cage, remove the rear cover of the spindle and blow clean any dust and or particles you may find.
You can find more information on regular spindle maintenance here including how to change brushes.