In cgminer,  any new block or any update of cuurent block will create a new "ntime" when block information is received.  About 60 second, block update information is coming from pool.
see parse_notify() function.

In cgminer, nonce2 is used to create more local block header data for more hashing and is used for generating new merkleroot in local header data. it changes very quick. see gen_stratum_work or gen_solo_work funct.
It is used in submit work to pool and pool uses it to re-generate block header for verifying if block header is resolved.