What is the lifespan of ultrasonic transducers like the ones used in hobby projects?
Ultrasonic transducers that use piezoelectric ceramics can be expected to have long lifetimes. These ceramics do age but in a logarithmic way.  Thus a parameter, such as the capacitance of the transducer,  might change 1% from 10 to 100 days but will change 1% from 100 to 1000 days and so on.  By the time that you receive the transducer, its age will be enough so that this time aging will not be of concern.  Otherwise, if its construction is good and it is not subjected to excessive temperature or voltage, it should last essentially forever.  I worked with sonar transducers using such ceramics that were expected to last at least 20 years.  The only failure modes that I saw were water intrusion due to failed seals, connection failures due to poor soldering and cracking due to excessive stress.