actually its "number of wallets with balance"/2^160.
You don't need to find a specific private key, anyone will do that yields a public key that hashes to the BTC address.
You don't need to find a specific private key, anyone will do that yields a public key that hashes to the BTC address.
That means the actual probability would be about 1/(2^135).
At a million hashes per second, that means you would likely find one after about 10^27 years.