How long will the battery last once the ALT failure light comes on? It depends. I'd say between 15 min and 60 min
On the Astros, in the UK at least, the POH had the number of minutes the battery would last assuming shedding all non essentials bar radio. It certainly turned my one Alt failure from an emergency into a no event. Does anyone know why this was discontinued? I accept it was an estimate but mine was 38 minutes with just over 20 minutes flight back to engineering late at night with nowhere closer open...