Too many factors to generalise.
For a start - what type of battery - conventional flooded cell, sealed conventional, AGM, Gel and within these groups there is a vast range of price, quality and manufacturers recommendations.
For instance, if you run a battery "flat" and each time leave it discharged for a week, then the battery life will be much different than if you completely charge it up again immediately. Rate of discharge is another factor, as is whether you really do recharge it to close to 100% or just recharge it until the idiot lights claim that it is fully charged. Overcharging with resulting loss of electrolyte level to the extent that the plates are exposed is another big no-no. How are you charging? Cheap taper charger that may actually overcharge, split charging on the vehicle which often leaves the battery chronically undercharged, big Sterling DC to DC charger that might exceed the maximum allowable charge current (bigger is not always better), or quality three-stage mains charger. Discharge rates? Absolutely essential that cook prepares a three-course meal on the microwave operating off an inverter powered from a single 80 Ah battery? Then expect an early demise of the battery.
Then there is "luck" involved as to how long the battery lasts before it fails due to natural causes regardless of how it is looked after - and of course the biggest factor of all - people skite about how long their batteries last just to make others feel bad.
Very hard to answer as it is a while since I had boats, but they certainly didn't have the charging systems that are around today. All I ever had on boats were simple split charging systems, sometimes with a tiny 10w solar panel for trickle charging. We used to go out on the boat every weekend and as long as the starter battery was ok, we knew that by the time we reached our destination we would have power in the leisure battery.
We were fairly ignorant of battery abuse back in those days and so we were probably on a wing and a prayer a lot of the time without realising it. We always assumed that the tiny solar panel was keeping us topped up, though in reality this was probably not the case.
Mind you, back in those days, all we needed electricity for, was lighting and occasionally a small TV. In all, I had 13 boats and I can only go by receipts or word of mouth from the vendor as to the age of the batteries, but as a rule of thumb, we used to expect 3-4 years.
To be honest, I am not technical enough on electrics to know the optimum setup and type of charger and battery, so I leave it to the experts to provide me with the best system, although, even the experts opinions conflict.
I see no mileage in 'skiting' though?
