Are you asking what are the possibilities of successfully finding the private key of an address by brute forcing? If so, as @hosseinimr93 said, 1 in 2256. You can try calculating with custom computational power and see what are the odds.
Say someone was giving you clues to their private key. For each character they gave you a couple of options eg, for the fifth position is either 1, 3, f or a. How many options would they have to give you per position for it to be crackable by brute force?