Obviously, the colder the outside temperature and/or the higher the water temperature in the heater, the greater the heat loss. A direct fired water heater can lose 8 degrees per hour when really cold. A direct vented water heater will have heat loss as well (maybe not quite as much?) unless there is insulated venting with a damper (which I'm not sure is available or practical).
My thinking without really investigating it is it would not pay to do this.