Motorcycle batteries have a more difficult life, due to vibration, long idleness and self discharge over the winter. If discharged, and left in that state, a lead-acid battery may not accept a charge. That's a frequent over the winter failure.
Automotive type chargers can easily overcharge the battery, another life shortener. A low current float charger for wintering charge can be a money saver. About $10 from Harbor Freight.
For some general motorcycle battery info, See -
http://www.mcnews.com/mcnews/articles/battery.htm
Re: your question on how long batteries last - I have some 3 and 4 year old batteries, still running strong. Don't know if that's good or bad relative to others, but I'm not complaining!