Well as long as your opponents are paying over the odds for their draws, then that's +ev for you whether they hit or not.

As a rough guide, go about 2/3 pot heads up, and full pot 3-way or more. If they are willing to pay that amount to chase a flush then the times that they miss will pay for the times that they hit. You aren't looking to push draws out here, just to make them pay more than they should. If they fold their draws to your flop bet then you've just lost value.

Go for as much as you think that particular villain will call given their range, but no more.