First of all we need to find out how likely we are to catch another heart on the turn. This can be done in many ways, but the most popular way is to find the ratio of cards in the deck that we don’t want against cards that we do want.
There are 5 cards in this hand that we know, our 2 holecards and the 3 cards on the flop.
This leaves us with 47 cards in the deck that we do not know.
Out of those 47, there are 9 cards that will make our flush and 38 that will not.
If we put this into a ratio it gives us 38:9, or roughly 4:1.
Now we know that the odds of hitting a heart on the next card are 4:1 (our card odds). This means for every 4 times we don’t catch a heart, 1 time we will.
Next we calculate the same ratio of odds using the size of the pot and the size of the bet. The link below has a great explainer and links to some material to use.
Our opponent has bet $20 into an $80 pot making it $100.
This means we have to call $20 to stand a chance of winning $100.
This makes our odds $100:$20 which works out to equal 5:1 pot odds.
Card Odds: 4:1
Pot Odds: 5:1
This means that we should call as the odds we are getting from the pot are bigger than the odds that we will hit our flush on the next card. In the long run we will be winning more money than we are losing.
Remember! You should only call if the pot odds are greater than the "card odds" (odds of completing your draw).
If finding the card equity by working them out in your head is too time consuming (which most beginners will) . You can find them more quickly by using odds charts. These are handy if you print them out and stick them next to your computer and refer to them the next time you end up with a draw.
For a longer (and better!) study of the subject try the Cardschat training linked here > Training
The simple answer is, yes. I will call to the river, if I am getting the right odds. But that is only because I'm not in the habit of calling pre-flop with any two cards, let's say 9-2, just because they are suited. If I am drawing to a flush, it likely to be with a suited ace or suited broadway cards, like K-J suited, so that I am drawing to the nut flush, or the second or third nut flush at the very worst.
Even then, I take into account the odds I'm getting before making the call. But what I often see is players completely disregarding the odds, shoving all-in or calling an all-in on a flush draw with something like 6-3 suited. When it comes to chasing a flush draw, I always consider pot odds, just as CowboyStomp laid out. When it comes to calling another players all-in, I never assume that I have nine outs remaining in the deck. I am mindful of the fact at a nine-handed table, sixteen other cards were dealt to the other players. And if one out every four of those cards were to be a heart then I could be down to as few as five outs. That is why I will make reasonable calls, based on pot odds, but am not fan of calling off all of my chips or shoving all-in with a flush draw.