According to this Guessing private keys
The the odds of guessing a randomly generated Ethereum private key is
1 in 115 quattuorvigintillion. (Or, as a fraction: 1/2^256.) That
denominator is very roughly around the number of atoms in the
(if you have 12-word mnemonic, the fraction is 1/2^128)
So this means the probabilty of guessing a private key is almost 0.
BUT if you save your key or mnemonic in your computer or your phone. Someone can still break into your computer or phone and find your mnemonic there. So there's always a chance that the place you saved your mnemonic get hacked. We can now say that the chance of not getting hacked is not 0 . Unfortunately, there is no certain number to declare the probability of your computer ( Or phone or anywhere else you saved your mnemonic) getting hacked.