My battery is 4 years old and I only drive my Boxster around 6 months a year as I leave it in Florida during the summer. When the car is in storage I disconnect the negative terminal as there is no electricity in my storage unit that would allow me to put a minder on the battery. When I take the car out of storage it starts right up so it appears that the battery tolerates this regimen. My question is: What sort of battery life might I reasonably expect from a batter subjected to this usage? Thanks for your thoughts.

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.

Disconnecting the battery may erase the emission readiness monitors so it won't pass emissions testing. Areal pain to reset. You have to drive the car at certain speeds for certain lengths of time to reset them. Each make of car is different. Instructions are on the net.
If I recall correctly BMW and Porsche are the same procedure.

Ed B