I don't think there's a turn limit; but if it's on two bars, I think it's best to throw a ball. It really depends what the capture rate is; but if you attack or evade you only have a 1/3 chance of leaving it on 1 bar (assuming you've got enough health left to attack), a 1/3 chance of killing it/letting it flee and a 1/3 chance of doing nothing/getting hit yourself whilst it evades. If the capture rate is less then 50% at two bars you should evade, and if it's more you should try and catch... I think. And I think it's more than 50%.