As a tight-aggressive limit player, I don't chase too much of anything, but when I do, it’s only when I have the odds, and sufficient reason to believe my hand will be good if I make it. Generally speaking, I will draw to a flush if I get the odds and there is not much betting going on, which suggests I'm up against a larger draw, or other hands that suggest a lot of lockout cards. A lockout card is a card that completes your draw, but leaves you with a losing hand. For example, when chasing a flush against a set, a lockout card is one that gives you the flush, but pairs the board so you’d lose to a full house.
There's no golden rule here except using pot odds, and that applies to No Limit (NL) as well. In NL though, chasing flushes is different. First, you have a much higher potential for implied odds if you hit your flush. In limit, you might be able to extract one or two more bets on the last street(s), but in NL you could take a healthy chunk of your opponents stack when you hit your draw. This means that you may be justified in taking a card off when you have less than ideal pot odds on the flop – within reason and with reference to your expectations about what you'll get on later streets if you hit your hand. You should be reasonable and err on the side of being too conservative when it comes to your NL implied odds calculations.
The other difference with NL is the huge issue of reverse implied odds. Imagine that you chased the king-high flush draw and one of your opponents had the ace-high flush draw, and the flush hit. While you were counting on a big pay-off if you caught your flush, you'll be the one paying off big because you hit your flush. This is true of limit, but only for an extra few bets that you will pay off if you chase a dead draw. In NL, you're looking at paying off your stack or a significant portion thereof. This is why you should be cautious about drawing to low flushes. If you get a great deal of players seeing the later streets it is important to decide if any of these other players have a better draw than you. If you're three-way or heads-up, then you probably only need to evaluate your own outs (less any lockout cards), and not worry nearly as much about reverse implied odds.
I'm sure you're familiar with the principle that chasing flushes on a paired board is a bad move (most of the time anyway), and this always applies as well.
Just remember when you decide whether to draw to a hand (on the flop) that you consider at least the following:
Am I getting the right pot odds to call? What about with implied odds less reverse implied odds?
Am I counting your outs correctly? What about lockout cards? How likely is it that someone else has a higher draw? For these questions, you need to think about your opponents’ hands, not just your own.
If I miss my draw on this card, how likely am I to get a cheap ride to 5th street/the river? What chance, if any, is there of a free card on the turn?
What are my prospects for winning the pot without making my draw? Does my opponent have a strong holding that they will not fold, or can they be bluffed? Can I make a better pair than they have to win the pot that way?
There are other things to think about, but generally there's no strength of flush draw that is playable in all situations (even the ace-high draw has to be folded many times, right?) or one that is never playable (I'll draw to a flush with 54s if I get the right odds in the right spot).
