How quickly will a car battery run out?

[stockhausen]

If a car (Mk. 5 VW Polo GTI) is parked in the street outside my house, with only the burglar alarm using the battery, how long will it take for the battery to lose power to the extent that it will no longer start the car?

 

[stockhausen]

There are a lot of variables. . . .

True, I just wanted to get a feel for how long would be likely

. . . I would advise against letting your car sit for that long unused.

Why so?