Question “How often does thermography have to be done on our electrical system?”
Firstly, In most countries thermography is not a legal requirement, however there are other reasons that it should be done.
Some insurance companies require thermography to be done as part of their insurance requirements. They do this to mitigate losses. Their requirement may specify an interval, this is usually annually.
However, there are cases when more frequent inspections may be required. Higher voltages, and more critical equipment may require an inspection every six, or maybe even every three months. Also facilities with substantially different summer and winter loads should consider a survey twice per year.