I think that's a place to begin. We'll need to experiment a bit, probably, to see how fast you need it to act, but I think "when you first feel the sensation" is a reasonable starting place.

[He retrieve a small flask from the bag he's brought with him. The glass is semi-transparent, hinting at a liquid with a reddish hue and a slightly viscous consistency behind the cork stopper.]

This was originally developed to poison weapons used against mages, so unfortunately it was not developed with taste in mind. [Translation: It's fairly vile.] If you have the time, I might dilute a bit of it in advance. It should be stable in water, and I've heard it's easier to swig a large mouthful with a milder taste than manage a small sip with a stronger bite to it. But that's just for ease of use. In an emergency, you can take a small sip directly.