Three months is the standard and if the bristles splay before that it’s usually from pressure rather than hard water - let the brush do the work or drop the intensity. Use the fading indicator bristles as your cutoff and don’t stretch past that, because worn tips clean poorly and can irritate gums. For hard water, a quick weekly vinegar soak followed by a thorough rinse and upright air-dry helps, and rotating two heads (so one fully dries) or buying soft off-brand multipacks can keep costs down.
