Not to minimize the other answers (which are completely valid) but just trying to give a bottom line quick answer (which I will admit can be misleading):
A big portion of your answer lies in these three items -
1) What kind of flush are you chasing? Are you holding a suited A? That's almost about the only time I will chase. Much less likely to chase if I'm not going after the nut flush or if I only hold one card of the suit in question.
2) How much is in the pot and how much do you have to call to see another card? What are the pot odds? Assuming you hold 2 suited cards and 2 of that same suit came out on the flop that means you have 9 outs to complete that flush. That means you have about a 35% chance to hit that flush within the next 2 cards, or it will happen for you 1 out of 3 times, OR 2 to 1. That pretty much means you can correctly call any bet that is anything less than or even up to pot sized to chase that flush. This just applies to the flop - do some homework to find your way after the turn.
3) But what if there is a pair on the board? That is a full house alert that you need to pay attention to because that will crush your flush. Be even more diligent if the bettor continues to give you odds with his betting.