How long can a battery possibly last

if you got a good tune and actually drive it, a battery should last for quite a while. My slant started on the first crank with a proper choke and carb. I mean it didnt go more than about 1/2 crank revolution stone cold. if thats how it would start forever, then there is very little draw on the battery as its always charging when the motor is running. if you dont go beyond your amp rating of your alternator, its just a matter of the battery breaking down chemically. the new AGM batteries (Yellow tops included) should be very good. I had one of those "it lives" batteries...it was a green top/white case national warranty battery you always see the trucks driving around. Cant remember the name now! It finally fell off a shelf and the vents popped off and it drained itself. Suicide? Float chargers are a must for stored cars. Every time a battery dies, it loses 10% of its cranking amps capacity upon charging according to an old mechanic I knew unless its a deep cycle marine.
We have huge clear Exide batteries in our telephone central office in series and they last forever. Once a week they are hydrometer tested and adjusted with acid/water and are constantly monitored electronically. As big as stereo speakers.