ISO15858 states: 4.4.5 Measurements are performed at initial installation; whenever new UV lamps are installed
(newer lamp designs may provide increased irradiance), and whenever modifications are made to the UVGI system or room (e.g. adjustment of device height, location or position of louvers, addition of UVabsorbing or UV-reflecting materials, room dimension changes, modular partition height changes).
In practice this will be when lamps are changed.