It depends on the tube’s heating time, on its brand, on its age and on its power. Some UVC devices that use 36/40 watt germicidal tubes recommend a range of about 3 meters.
To further reduce shadows, you can remain in two opposing sides of the room and irradiate each side for 2-3 minutes. Circulating air (with a ventilator) while a UVC device is running will even help to irradiate air from all areas. UVCs at this power remain the fastest method of decontaminating rooms.