Modern batteries....the ubiquitous Johnson Controls batteries, which use lead-calcium construction lose charge very slowly when unused. I generally find our boat batteries (our watershow team uses a total of 9 JC batteries to power and start our boats) are nearly fully charged after 7 months of storage each winter. Well above 90 percent charged. I only use "car batteries" the warranty is much better!!
The triple rig has 3 batteries from 2007, it again popped right off this spring without any recharging. A battery on a store shelf should easily hold a nearly full charge for many months.
Exide: This struggling company still uses lead-antimony construction, same as the batteries our fathers and grandfathers bought. These typically lose 1 to 2 percent of their charge per day, and likely should be charged if they have beeen on the store shelf for a couple of months. Perhaps this was the source of the old superstition that a cement floor sucked the charge out of a battery....yeah it went dead sitting on the floor, but also went dead if it was anyplace else also!
Note this conversation does not apply after the battery is in the car, some cars have terrible parasitic drain even when the car is seemingly off. Also, many Mercury Outboards have computers that also drain the battery even when the key is off. (Our team only uses Evinrude ETECS, they have no current draw when off)
For those of you unfamiliar with battery chemistry, small amounts of either antimony or calcium is used to alloy the lead plates so they are not way too soft and weak.