How long is normal for Boxster to sit & battery still work?

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If the battery is more than 5 yrs old, replace it. These cars do have a small parasitic battery drain while at rest especially if left unlocked. If you leave the car unused for several weeks often, put it on a maintainer or add a battery disconnect switch.

Another thing that can kill a battery is a sticky switch on trunk lamps. Both my front and rear trunk switches got fluky over the years so I just pulled the trunk lamps and that solved the battery drain problem. If your car has a glove box lamp, it too can stick closed and kill the battery.