Philips provides a warranty for each Sonicare toothbrush, so the minimum lifespan of a toothbrush coincides with the warranty period for the specific product, which is either 12 or 24 months, depending on the model. However, the actual life expectancy of the toothbrush may be longer than that.
The actual life expectancy of any product depends on the life expectancy of its parts. For example, the Philips Sonicare line of toothbrushes use nickel-cadmium batteries, which can last over 1,000 charges. Philips generally recommends charging the toothbrush every two weeks with regular use, so the battery alone could last many years. However, the button that controls the brush head could fail more quickly.