There are three different metrics used in cryptanalysis:
The amount of time required to attack a cipher may be so prohibitive as to make the enterprise infeasible.
Advances in computer architecture may increase the amount of work that may be done per unit time.
The amount of memory required must be considered as requirements may be so large as to make the attack infeasible.
Advances in computer architecture may make larger memories more accessible.
The volume of ciphertexts required may be so huge as to make the attack
L. R. Knudsen, M. J. B. Robshaw, The Block Cipher Companion. Springer, 2011.