As the questions says, when do you stop? your point of no return?
d. you stop when you are at least break even
I think this is the closest answer that you provided OP. Let me help you and add another choice to your question, which is as follows:
e. When you at least profited even the smallest win.
From my quoted answer above, I usually stop gambling whenever I have profited the slightest. Even if the profit may be relatively small, I take all the small profits as a big win on my part given the risks associated with gambling. On the other hand, if I have exhausted my allocated budget in gambling without even winning, then I take that as a sign of defeat and consequently retreat from my position.