Open Ended Straight-Flush Draw

A standard open ended straight-flush draw would be 15 outs

From the flop (2 cards to come) the odds are 54.1%
From the turn (only 1 card to come) the odds are 32.6%

So from the flop you always have positive equity for any call

And from the turn it would have to be a bet of below the pot size to justify the call (bet pot would cost you 33%)