I consider anything longer than a month to be quite some time. I usually pass on riding during the winter season.
A great feature of the charger I mention above is that it will only charge if the battery is below a certain voltage. So I tend to plug the charge more often than not.
If the battery is fresh or a great brand, you shouldn't really have to worry about the battery losing voltage. Six months can go by, and it should still be fine. But for batteries that have been used for a few years, this will definitely extend the life.