I have done a cursory web search and have found nothing definitive or absolute to answer your question.
Some nurses recommend changing only if the pad loses contact with the skin which would interfere with transmission of signal.
Some recommend changing every 24-48 hours.
Any time the leads/pads are changed, position should be changed to allow the skin under the previous pad a break from having adhesive on it.
Yet the pads should not be moved greatly from their previous position, so the ECG tracing remains as close to the original as possible, to provide continuity in monitoring rhythm changes.
I work in a cath lab and when patients arrive in our lab we routinely remove all the pads and apply new ones so they are not in the field of view when we take the cath images.
I see a LOT of red and irritated skin from either patient sensitivity to adhesive or from electrodes remaining on the skin for a long time.
Therefore I endorse the "change every 24-48 hours" rule.
Look up proper skin prep too, before electrodes are placed. This will impact the quality of the ECG tracings.
Hope you get more responses, sorry I couldn't find a hard-and-fast rule.
Maybe someone will do a project???