There is no easy answer to this question as the answer depends on many distinct factors. Not only must the work and computational resources required by the cryptanalyst be reasonable, but the amount and type of data required for the attack to be successful must also be taken into account. Furthermore, the value of the concealed information must be taken into account - it is reasonable to spend a million dollars of effort to uncover something worth more than a million dollars, however, any sane attacker would not, for example, invest one million dollars to uncover a secret worth one thousand dollars.
Also, it should be noted that cryptography and security are not equivalent. If a block cipher takes seven months of computational effort to crack, but the key can be recovered by bribery or extortion, a truly dedicated adversary will probably attempt the latter.