I'm willing to test, but I can't seem to google the means to determine a NPC's health. It could be as simple as opening the console and clicking on someone, but I'm not in a place where I can check.
As it stands, level 40 or 41 and Deathlords take around six seconds to kill (although I tend to use the lower powered Fireball spell more often than incinerate.)
I kind of agree with sh00byd00, but since you seem interested, the way to determine an NPCs health is by entering the console, selecting them, then typing:
getav health
getav stands for "GetActorValue." There are a bunch of actor values, skills are things like heavyarmor, destruction, lightarmor, onehanded, sneak, etc. Health, magicka and stamina rates are :
healrate
healratemult
staminarate
staminaratemult
magickarate
magickaratemult
The list will get a lot more comprehensive, and be available on the CK wiki, once the CK is released.
*edit - if you want their maximum health, type:
getbaseav health
or
getavinfo health
for a more comprehensive list of both the actor value and the modifiers effecting it.