Are we talking tournaments or ring games?
In ring games, its easy:
This is the classic example of when pot odds
become important. If you have 2 to a flush on the flop, the chances of landing a flush is
1 - (38/47)*(37/46) = 34.97%, which is 1.86:1 odds.
Therefore, if you are getting more than 1.86:1 pot odds, you should call.
If there is 10$ in the pot and someone bets 5$, you should call because you are investing 5$ to win 15$, a 3:1 payout. If there is 10$ in the pot and someone bets 20$, you should fold because you are investing 20$ to win 30$, a 1.5:1 payout.
The equivalence point (which has almost no practical value in poker) is when they bet 11.63$ into a 10$ pot, where the expected value
of calling and folding are the same.
So, as always, it depends