> The exact lifetime depends on how much current the pacemaker takes to operate.
Not so sure. I don't think the plutonium decay is affected by the current drawn via the thermocouple - it'll just keep putting out the same power (well, the same decaying power curve anyway) no matter how much current you try and draw. I suspect the thermocouple's voltage will just sag so the maximum drawn power will be the thermal output less the efficiency of the conversion (which, I guess, might be non-linear with current?).
But I'm pretty sure whether you draw zero Amps or short circuit the output, you'll still have precisely 50% of your plutonium left after 88 years, right?