Given a 128-bit key used for authentication based on AES-CMAC, the NIST 800-38B recommendations suggest at least two criteria for a good key cryptoperiod:
- after 'MaxInvalids' error messages the key should be retired (considering the MAC truncation and the accepted Risk in appendix A);
- the key should be changed to limit its message span (e.g. $2^{48}$ usages for AES-128).
My question is: should the second criterion be considered if I have MAC truncation? In my scenario, I'm assuming that the attacker can only see truncated MACs (suppose $30$ bits after truncation). Since I'm using truncation, and this criterion is made to avoid attacks based on detection of a pair of distinct messages with the same MAC before its truncation, should I still consider this criterion? If yes, should the limit still be $2^{48}$ or is it affected by the truncation?
UPDATE
The scenario is: only truncated MACs are sent to a limited-bandwidth network, and the attacker has access to the network. Considering Elias answer I removed the assumption that the attacker sees only truncated MACs, which is not meaningful. So, if we remove this assumption, do I have to shorten the cryptoperiod to consider $2^{15}$ (truncated MAC), or I can just consider $2^{64}$ (entire MAC)?