The TVS doesn't blow because of a voltage spike, unless it's a really big one that exceeds it's max rating. They will degrade over time, but it will be years before they need replacement. The only way to really test them is with a fairly complex test setup, so if you're nervous, just put a date on the install and swap them every three years. 
You want the TVS across the power as the last thing in the circuit, past any circuit breakers.
