You say just for fun, I wonder where is the fun if we start mining with our CPU 24/7 and never mine anything, could you provide some probability estimations on a single block found/time?
I've just fired this up on one core of a i5, running through 10MM hashes in around 56 seconds. That gives me 178,000 hashes/s or 0.178M/h, which presently equates to 65 BILLION years to find a block. That's between 14 and 15 times the age of the Earth, and slightly less than 5 times the age of the universe and everything in it.
Still, gotta be in it to win it!