[adanthar]

Depends on how many outs you have (does he have KQ or QJ) but either way you've got at least 12, so just under a 50/50 chance of winning, the pot is 2100 counting his bet and he's saying he's weak. You basically have to go all in to maximize folding equity, but even if he calls every time, you're risking 1500 to win 3500. To make an accurate fold here you'd have to know he has a set, and then it would still be close.

Note that him going all in instead of betting 300 changes absolutely nothing. It doesn't even matter if you have T9s because if he goes all in you still have the odds with only 9 outs. So in a really big pot, or with a big draw, always going all in no matter what isn't ever really wrong except for weird bubble situations.

If your draw is smaller, or especially if the pot is smaller, though, things change. Assuming the pot is 600, you have T9s and he bets 300 and flashes a Q, you should just call - he's not folding to an all in and if you add up both your stacks you barely have the 2:1 you need. If it was a bubble situation and there was a really short stack around, occasionally you'd have to fold depending on the other stack sizes.

Basically, the answer tends to change based on the pot size and the number of players/chips. When there's 6-8 players left or more, though, you may as well just pretend it's a ring game.