the only way to be sure !
Following a major avionic upgrade on my Robin I checked the life of the battery first the whole electrical system was run for 5 min and then the system was "load shed " leaving only the HSI , GPS , NAV/COM 1 and the transponder running in avionic stack.
The nav and cockpit lighting was running along with the T/C and all the cockpit indicators.
After 40 min RMI , ADF and DME were switched back on for 5 min and all functioned for the test.
The test was ment to simulate an alternator failure in IMC , the load shedding drills being done and then the minimum equipment being used for an instument approach.
The battery was fully charged and the OAT was about 5 C.
I would guess that the battery had about another 5 min before things started to fail and I suspect that the COM TX would be the first thing to fail.