grity, For springs not operating in the yield range, the lifetime is very long, in the millions of operations. For a dual failure under this condition, the probabilities suggest that the first failure must not be observed, and the system continues to be operated with a single cartridge providing feel. If this case is valid, then the probabilities of two broken springs on the same flight move into the possible range.
We have to ask the 'Bus maintainers and pilots, the likelihood of a single failure not being written up within a reasonable period of time ~ 6 months max.
Pilots might like a softer feel to the spring cartridge and might not write it up.
Maintainers have periodic checks that they do on everything. The question is not how often this is checked, but how often is it checked using a method capable of reliably detecting a partial spring failure.