I have an 86 944 turbo with 180K miles on it. It sat all winter (outside, upstate New York) with the battery in it. When the snow cleared enough for me to open the doors (March 15), I put the key in it and it started right up. Keep in mind that this is a relatively new battery with a full charge in November.
If you have a problem with battery drain, hook up an ammeter and check the draw when the car is sitting with nothing turned on and no key in the ignition. You should be drawing less that 50 milliamps. If it is more than that, use the following to diagnose:
Excessive Battery Discharge Rate