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?
Well, we can work out a rough idea. Let:
x = number of addresses you can derive from their respective private keys each second
y = length of time in seconds you are willing to wait
Given that a private key has 64 characters, then the number of possibilities for each character would then be given by the 64th root of the product of these two numbers, so
64√(x*y)
For example
Let's say you are running a GeForce RTX 2080 Ti, which can turn 2.5 billion private keys in to their corresponding addresses each second, and you are willing to let this run for two weeks. 14 days * 24 hours * 60 minutes * 60 seconds * 2.5 billion = 3.024*10
15 private keys. The 64th root of this number is 1.745, so you wouldn't even be able to exhaust the search space of 2 characters per position in 2 weeks.
2
64 is 1.845*10
19, so again running your GeForce RTX 2080 Ti and checking 2.5 billion keys per second, it would take you ~234 years to check all combinations, or ~117 years if you wanted a 50% chance of finding the correct key.