Roughly speaking:
Betting on the flop is -EV unless you go all-in and are called by at least two opponents, or you are called by at least 4 opponents.
Betting on the turn is -EV unless you've hit the flush or you are called by at least 4 opponents.

Now, there are tactical and strategic reasons to bluff or semi-bluff, so making -EV raises here isn't always a bad idea, but doing so frequently will loose you money.