The formula used to measure the time needed for computing power to advance sufficiently
to feasibly brute-force keys of various lengths is described in
a 2016 post,
excerpted here:
"...the maximum possible decryption speed available today would be able to brute force a 64 bit key in a second.
Moore's law says that computers get twice as fast every 2 years.
In cryptography terms that means that advances in computer power will give you one extra bit every two years."
For the purposes of the Time Machines project, a cipher image is deemed vulnerable when it becomes possible
to crack it by applying the
"maximum possible decryption speed available" for one month.
Moore's law is usually understood as applying specifically to integrated curcuit technology, but it is often also viewed as representative of progress more generally. In the longer term, it seems likely that computation will become vastly more capable, even if current technology becomes obsolete.