you have about a 20 percent chance to hit your flush on the next card so if you are being offered pot odds of atleast 4:1 you should atleast call. example your opponent bets 20 dollars and (with his bet) there is now 80 dollars in the pot. You can call with worse odds if you expect that he will call another bet from you on a later street. If you expect to see more than one card, for example, he is very passive and will almost always check the river if you call his bet on the flop, then you can call again with worse odds. You are like 2.7 to 1 against to hit if you see two cards thats about 37 percent.