Keys, once used in the production movement, will never expire. However once the keys are issued, bank should go live in production within the prescribed TAT of 90 days; if not done, the bank needs to procure new set of keys from NPCI.
The Switch Keys depend on the ASPs. In case you have the same ASP for both Acquirer and Issuer, then the same set of keys can be used. In case the ASP is different for the Acquirer and the Issuer, then each ASP will have a different set of keys.
NPCI is not responsible for loss or compromise of keys after received by custodians. It is the Bank and the custodian's responsibility to keep the key safe and secure.