For me it would be some months after the Ethereum switch to PoS. People will start looking to ETC seriously, and the price, as well as the hashrate, will severely increase. My guess is March 2018.
The miners should not able to mine ethereum again and etc will be the alternative source for them, More miners will migrate to the ethereum classic. But the price will increase like what was happened with ethereum itself .
I think most miners will only mine the more profitable coins.