My 1999 +8 will discharge a fully charged battery in about 2 to 3 weeks if left alone with no use. I have always assumed that this is the immobiliser circuits, but don't have any measurements to quantify this.
If I isolate the battery from the all the circuits (except my SatNav charge point) by removing the red isolation key , it keeps its charge for considerably longer (again not quantified but would suggest at least 2 months) However when I do this it appears to take a few miles before the ecu "relearns" my driving habits, so I don't like this a method of preserving battery life.
Like Dave W I keep the car on a trickle charger all the time it is in the garage. That way I have not had any problems.
