The BU 550 lifespan is meant to be very long.
The components which are responsible for reliable measurements are carefully protected.
During the development process of a new product the concerned components run through certain stress tests which could be dominating throughout the lifespan. We usually check if those components are suited to "survive" the 5 - 10 year usage under "usual" conditions.
We definetely reccomend, after the international standards too, to let your device be tested by the specialist supplier every two years to guarantee an exact measurement process.
If you proceed after this rules and after in the user manual listed advice your MEDISANA device has to possibility to "survive forever". This depends on how the users treat the product. We can never guarantee that your device's lifespan reaches ten or twenty years because it contains a lot of electronical components which could fail under different circumstances.
Please note that the cuff is a wear part and can develop leakage if it is exposed to heavy stressed situations.