You could use the Pre and Post render MEL options in the Render Settings -> Render Options to get the precise render time but i think thats nothing for batches because the command line renderer doesnt spit out print code for each frame startet from there. Instead, print the render times to a file. Therefore type
into Pre render MEL: global int $renderTime = 0; $renderTime = timerX
;
into Post render Mel: getRenderTime();
Into your Expression Editor start a new Expression and copy into it:
global proc getRenderTime()
{
global int $renderTime;
string $mode = filetest -f "c:/renderTimes.txt"
? "a" : "w";
$fileId = fopen "c:/renderTimes.txt" $mode
;
fprint $fileId("render time: " + timerX -st $renderTime
+ "\n");
fclose $fileId;
};