Originally Posted by graraps

That's not necessarily true, indeed I believe it not to be true for most cars. I used to disconnect my car's battery and leave it for months, then I'd come back and it would be running smoothly from the first second. I only had to set the time/date and make do with the trip computer not holding the older data.
If you want a general cut-off point, I'd suggest 7 weeks (providing your battery is in reasonably good condition).