How many buy-ins must you lose before calling it quits for the day?

Assuming you all play with a minimum of 20 buyins of the stakes you play at, how many buy ins must you drop in a day for you to just "X" out and call it quits to avoid tilting and losing bigtime.

The biggest loss I have ever had in a night was 6.5 buy ins and I regret not stopping earlier. After reviewing my play of that night I noticed that after a certain point I was making extremely weak-tight plays that cost me pots left and right. Had I stopped after my 3rd buyin perhaps I wouldn't have dropped so much.

But what is YOUR personal limit?

Thanks guys