To keep a battery in good shape, the car should be started once a week and
the engine run at speeds higher than idle.  While the engine is idling, the
alternator barely puts out enough amperage to charge the battery.

A brand new battery that is not connected will last several months and still
be good.  If it is hooked up to a car, parasitic draw from the radio, clock,
and computer will probably drain the battery enough that a car won't start
in two or three weeks.

If the car is kept in a garage, think about installing an automatic trickle
charger in the car.  The charger is installed in the car and all you have to
do is plug it in when the vehicle is parked for a long time.   The charger
starts and stops as needed to keep the battery fully charged.

 If the battery is installed in the car and hooked up (running the
clock, the ECU memory circuits and a possible burglar alarm system)
you should let it sit no more than two weeks.

 And every week to two weeks you need to take the car out for a
half-hour to 1-hour drive to recharge the battery.  

 You might get a car without an alarm or any other added loads to
start after sitting unused for a month or more, but the small draws
from the clock and the memory will be deep discharging the battery
over that extended period and causing cumulative battery damage.  That
6-year battery might only last you two or three years.

 If the car is regularly sitting for a week or more, get either a
solar cell trickle charger or mains-powered trickle charger.  If you
use a 120V charger, be sure to drape the extension cord over the
drivers door or the windshield, so you don't forget to disconnect it
before driving off.

 If you park in a garage, a solar cell is obviously a bad choice...

 If the battery is sitting on the shelf at the parts store, or in
your garage, or in the car but the cables are disconnected, you need
to put it on a trickle charger to top it off every three to six
months.

 Unless you have a very unusual size battery they don't sell many of
(example the VW Beetle with the special cover tab over the positive
post and the remote drain/vent hose) I wouldn't buy a "new" battery
with a manufacturing date code over six months old, because I can
guarantee you stores do not recharge the batteries in stock.  

   --<< Bruce >>--

 A battery maintainer is what you need.   It will not boil the battery
dry like a trickle charger.   Google battery maintainer and you will
find what you need to know and buy and use.