It is recommended that you replace your Philips Sonicare brush head every three months of normal use (brushing twice a day) or when the blue indicator bristles wear out. One reason is because brush heads can become worn and become less effective at removing plaque.
Note: Some Philips Sonicare toothbrushes have a brush head reminder feature. The replacement icon on the handle will start to flash when it's time to replace your brush head. You can expect the reminder to come up approximately every three months of normal use (brushing twice a day for about 2 minutes).