Until an explosion and no insurance claim.
Read on.....
BS-approved UDHW systems training manual, under the heading of maintenance, p40:
"the installer is responsible for the safe INSTALLATION of the unit. He/she must also make his customer aware that periodic checks of the equipment are ESSENTIAL FOR SAFETY... some manufacturers recommend a MAXIMUM of 12 months. Experience of local water conditions may indicate that MORE FREQUENT inspection is desirable."
BS6700: 1997 states that with unvented cylinders, the "maintenance and periodic easing of temperature relief valves is particularly important".