I actually have a data point on that. It all depends on the SOC% before being disconnected from grid power.
I had a Founder's PW installed at our last house, then we moved. I had it uninstalled by Tesla (at 100% SOC), and it then sat in the basement of our new house for 19 months before I could get it re-installed with our new solar panel install. We were all afraid it would be dead after sitting for so long.
When they got it hooked up and running -- the SOC was 97% and has been running perfectly since June.
In another thread a former Tesla PW engineer said the shelf-life when new was about a year, but that's only because they are shipped at a 25% SOC.