3 days is definitely not good for an NC. That said, mine is usually around 2 months before the battery is drained. That should give you some better expectations.
The next time a mechanic tells you cars can't sit for a few days without a battery drain problem, ask how people who park at the airport get home after a two-week trip.
the real key to long storage (beyond a trickle charger) is clean gas.
right now, though, it only has to handle 2 weeks of sitting at most (in an attached garage).