It depends: environmental conditions, quality of battery, storage, condition of the bike, etc.
My last Yuasa AGM battery was never on a charger, lasted 7 years, and would sit 2-4 months (November - February) with no problems starting the bike on the first push of the starter button. However, that 2-4 months sitting was in a protected garage (underground) where the temperature was a steady 50 degrees Fahrenheit and the bike is in perfect condition. Also, I consider Yuasa to be a premium brand.
The replacement Yuasa AGM was shipped dry, so I purchased a float charger (Yuasa brand) for the initial charging. I now pull the battery (no outlets near the bike) and place the battery on the charger for those 2-4 months, but it sits in the bike the rest of the year when I ride 2 - 4 times a month. Again, always starts on the first push of the starter button.
My Bonneville has a parisitic drain (for the clock) so I'll hook up the charger every 4-5 weeks if I haven't ridden. On the Vulcan it was more like six weeks, but I rarely went that long with out a ride...
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