Your regimen makes perfect sense under the circumstances. With the battery sitting for long periods of time sulfation (buildup on the plates inhibiting charge) will be accelerated. You can ameliorate this and extend the life of your battery by getting a desulfating battery charger and occasionally using that function when your car is near a power source. Best not to wait until you see symptoms before doing this.
In terms of longevity under the current circumstances, I would estimate 6-8 years.