I know of no way to query it directly, but this solution works:
Put the following in your Pre Render MEL
(from the Render Settings):
python "global last_render_time;import time;last_render_time=time.time()"
Expanded for readability:
global last_render_time # not needed when in module
import time
last_render_time = time.time()
And put this in your Post Render MEL
:
python "global last_render_time;import time;last_render_time=time.time()-last_render_time"
Expanded:
global last_render_time # again, not needed when in module
import time
last_render_time = time.time() - last_render_time
This will store a global python variable last_render_time
which is the number of seconds the render took.