I'm sorry if I'm going ad-hom, but it's quite clear you don't know how encryption works.
You'd want a good 1000-10,000x factor of hardness over whatever you think is hard enough, otherwise it just gets easy to farm out the computation for something that would 'normally' take 10 days to a botnet that brings that down to 1 hour. For this reason you'd really you'd want to set the 'normally' to the whole of the earth's computation capacity, which has really exploded recently due to bitcoin.
Also it's not so easy to tune. The whole reason why we use the encryption algorithms we use is that their difficulty goes up generally at 2^x, where x is the size of the key. If you get an extra 10 bits in the key (bits, not bytes) you end up with a 1,024x harder problem.