I have been servicing and repairing Ultrasound devices in Physiotherapy for over 29 years now, over that time i have seen many makes & models. Some units (30+ years old) have very good outputs even today, (as long as they are serviced / calibrated each year) . The bond (glue) used to stick the crystal to the metal head plays a very large part in the quality of ultrasound transmission from the crystal to the head, if this is looked after, i.e not dropped or knocked then along with regular calibration they will work well and last for many years. However I have seen some very poor quality manufactured heads that certainly would not last more than a few years, luckily these are not in wide spread circulation.
The Ultrasound heads are matched to the base unit (driver) and when set up correctly, operate effectively. Out of calibration devices can cause the crystal to be driven at an unmatched frequency and therefore deliver a lower output. The most common cause for a crystal to fail is damage (a small blow to the head can cause damage even if it is not visible on the external surface of the head). We generally replace 1 or 2 crystal / heads a week, based on us servicing over 1500 per year in the UK.
So to conclude, I would estimate the average life of a crystal / head that is looked after (serviced yearly and not physically damaged) would be over 10 years. Please also note that crystals can be replaced, so the life of the Ultrasound can be extend (I still see machines that are 40+ years old working ) with the cost of parts being between £80 and £300 dependent on Manufacturer and availability.