My home is being baited for subterranean termites. How will I know when they're all dead?

The technician will assess the activity in the baits over the course of the program and, usually, make a judgement call as to when success has been achieved.  A good tech will almost never tell you that the termites are all gone but will talk about colony eradication or control.  You'll still need ongoing inspections.

Baiting is used for subterranean termites (not drywoods and rarely for dampwoods).  The big problem is that there may be more than the known colony in the area or that the colony has fuzzy boundaries and some may survive the poisoning.  A great example of this is from Devon, England where introduced termites turned up in two houses.  More than a decade of baiting later, there are still some termites active.