The average subterranean colony works for three to five years before anyone notices.
Termites eat wood from the inside out and leave the paint layer intact, which is why the first visible sign is usually a section of skirting or architrave that sounds hollow when tapped. Mud tubes on foundation walls, buckling paint that looks like water damage, and doors that suddenly stick are the other three signs worth checking every spring.
An annual inspection costs a fraction of a single structural repair. If you are buying, insist on a timber and pest report rather than relying on a general building survey — the two look for very different things.