This exercise is from Introduction to Modern Cryptography (2nd Edition) by Katz & Lindell

c.n (log n) The best algorithm known today for finding the prime factors of an n-bit number runs in time 2cn3 (log n,) Assuming 4Ghz computers and c-1 (and that the units of the given expression are clock cycles), esti- mate the size of numbers that cannot be factored for the next 100 years