based on number of outs?
9 outs for flush you get about 4:1 odds on each card after the flop or 2:1 total. Which means the odds of making the flush on the turn card are 4:1 same as making it on the river.
So on the flop if you are getting better than 2:1 Say the pot is $4 and the bet is $1 then you are getting right odds. Now on the turn say its $6 and the bet is $2 and you have not made your flush you are only getting 3:1
pot odds on 4:1 actual odds of hitting the flush on the river. So the call is not profitable.
If you are chasing one overcard (like an ace which many in limit love to do) you are rarely getting the correct odds. It's like 14:1 Plus your ace may not even be good (alread up against two pair or a set in some cases)
Two overcards its 7:1
For a quick reference here is a chart
Poker probability (Texas hold 'em) - Wikipedia, the free encyclopedia
As for open ended on paired board, generally no. But pot odds would have to offer better than 4.88:1 (according to that chart) and it would really depend on the situation. This is just me, others may say the best option is to just fold.
For instance
There are two limpers and I'm on the button with the 89s and I limp as well. The SB completes and then the BB raises. He's probably got a big pair or AK since he's pretty tight. We all call and the pot is at $10.
The flop comes 667 SB checks and BB bets $1 it's folded to me and the pot odds are a nice 10:1. I call
Hopefully the SB folds and its heads up. So lets assume he does
Bet on turn is $2 into a $12 pot with 6:1 pot odds you're still getting the right price.
I only call with the right pot odds AND if it's heads up. If it's more than heads up then there is an increased chance of danger. The raiser doesnt likely have 77 66 or 67 so you dont have to be that worried about him just yet. It's possible that you could get hit by a full house later in the hand to his AA or something but probably not. However one of the callers, if its not HU after the flop, could have a six, or maybe even a pocket pair such as 5s. (wouldnt like your 9 high straight then would you?
)
On the other hand lets say I have QJ and the board is KKT. I fold here even if pot odds are correct its just too dangerous.