How often do you have to fly a year in order to justify getting a miles card over a cashback card?

You don't have to fly frequently to get a miles card. If you get a card whereby the miles don't expire, you can slowly build up your stash of miles for a good redemption.

However, if you are not the sort to maximise category spending or find that you don't really have aspirations to fly in premium cabins, then perhaps a cash back card might suit you better, just make sure you waive the annual fee.