After the flop, there are 47 unknown cards (52 in the deck minus your 2 hole cards and 3 for the flop), 9 of which will make you the flush.

So the odds of making the flush on the turn is 9/47 (about 1 in 5)

The odds of NOT making the flush on the turn is therefore 38/47. If you don't make the flush on the turn you can still make the flush on the river. There are now 46 unknown cards (you also know the turn card), 9 of which will make you the flush. So the odds of making the flush on the river, if you don't make it on the turn, is 9/46.

The total odds of making a flush either on the turn or the river is:
9/47 (odds for the turn) + 38/47 (odds of missing on the turn) * 9/46 (odds for the river) = 0.35

So you have a 35% chance of making the flush on either the turn or the river.

If someone bets less than 1/4 of the pot after the flop I will call. I will sometimes call a larger bet if I think the implied odds are large enough. I also occasionally raise (semi-bluff) a flush draw.