We usually use the brush heads longer than the 3 months recommended by the dentist (once a day).
Philips still helps somewhat with the assessment with a few blue bristles that eventually lose their colour. If used several times a day, the bristles will definitely fade in less than 3 months.

The brush heads can be used for longer than 3 months, but their durability is highly dependent on the frequency of use and the way they are handled. If the bristles are frayed (like a "hedgehog") due to excessive brushing pressure, they can cause gum injuries and reduce brushing performance. Otherwise, I agree with Bruno.hengartner's answer.