I'm mining with 320 cores from the very beginning, got several zero-blockcs (7 or

, and 4 non-zero blocks. I was running without scantime and -a option, now will try them.
Pretty nice record. A friend of mine is mining with ~5000 cores and only got like 20 blocks, albeit he is mining from the onset of blocks with finite reward.
Cheers!
Edit: So far general consensus might be you need ~ 200 - 600 cores to find a block per 12h! WOW! I remember mining with laptop when XPM was released and still getting a block.