according to the Official Guide the drop rates for Burzzthak are 0.13% per item
find item gives +0.1% per point increase of creatures current drop rate, level 800 gives 80% higher drop rate. That's 0,234% drop rate per item.
two possible cases (i haven't studied the board enough to see if the answer is somewhere

)
* if the game first selects an item and then checks if you get a drop, you have a chance of 0,234% to get a drop when you kill him, around 425 kills for an item
* If the game checks every item for a drop util you get a drop or every item is checked you have a chance of 0,936% [**] to get a drop, around 106 kills for an item
with find item 1000 the same calculation would give around 384 kills and around 96 kills
[**] ok, that might not be entirely true, but it's close enough (if i remember my math it should be a little lower... since this calculation above adds the case when the first items drops and the second is dropped and so on, but you cannot get more than one drop at one time, so you woudl probably have to substract those cases to get the exact number... but then again i might remember wrong

)