I think it is highly dependent on the battery and the environment.
Get the battery fully charged, and record the voltage shown on the screen as the max voltage. Then shoot until it show battery warning or automatically shut down. This is the min voltage.
I think this is the best way.
I have bought GP ReCyko and they seem to be really good. I was out in -10C the other day and they performed really well. I would like to set the min voltage right so I can get the right notification when the voltage is low. How do I achieve that?? I have a Canon S5IS with the latest version of the CHDK.