I think as technology makes bigger strides the time-frame to access Bitcoin by cracking private keys becomes smaller. It is probably sooner than we think but there is every chance an update will be made to counter any threat before it arises. There is no reason crypto will not be able to stay one step ahead of any perceived threat.
In my opinion, the time expected for computers to crack down private keys is dependent on the computing power and since that is limited due to our current technology . not much could be said after that. What's clear, however, is that once one builds up or manages to access as much computer power as required to compute all the different pattern permutations the security will be entirely breached.
Though, this is requires an enormous amount of energy, and also resources we currently don't have, and are in the process of just about understanding what they are.
So it will take at least 10-20 years before private keys become useless.