What if you could just narrow it down a bit. Third character is in this range, fourth in this range, and so on. Everyone quotes the unsolvable probabilities but what is a solvable probability given computing powers and botnets these days? 2x? What is x?
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 2
160. You can try calculating with custom computational power and see what are the odds.
The problem is that you don't know which of those keys generate the same address. If you had a list of 2
96 numbers that generate completely different addresses, then yes it would be easier to brute force, but you don't.
Specifically, if you had that list and a machine that performs 100 trillion ECC multiplications and SHA256 hashes per second it would take you 79,228,162,514,264 seconds to get all possible addresses' combinations.
which is equal with 1,320,469,375,237 minutes, 22,007,822,920 hours, 916,992,621 days, 30,566,420 months, 2,547,201 years. And remember, you don't have that list of special numbers.