In Training with Power, Allan/Coggan give guidelines for when to stop the intervals based on interval length. The longer the interval, the smaller the margin. For instance 20 min intervals, they say quit once you've dropped 3-5%. For 1-2 min intervals, they say quit when you've dropped 10-12%.

I know for me, when doing stuff like VO2Max intervals the second to last is always the worst, and will be a bit low. But then I usually manage to dig in and get the power back up for the last one.