How long do charge a dead battery for a riding lawn mower?

till its charged...put it on slow charge and let it set the night. Here is the big secret on batteries....with both post exposed and not running, just the air afloat in the air will drain the battery, solution....when you are going to store for the winter, pull the battery out, put it in a ice chest, or sealed container so air does not get to it, and I think you will find next season it will still be charged.