How to calculate time needed for decryption of 64-bit key I am having trouble figuring out how long it takes to decrypt a 64-bit key, given that a computer can do 1 billion trials per second. I know that there are $2^{64} = 1.844 \times 10^{19}$ possible keys for this key size, but then how do I go about figuring out the rest? If there is a formula for this, I would be very curious to know what it is.
If you measure the number of keys you can guess per second (as suggested in the comments) you can calculate from there. As a frame of reference, if you can calculate 1,000,000,000,000 (1 trillion) keys per second, the time to break a 64-bit key would we an average of 15.25 weeks (106.75 days), with the longest possible time to break being 30.5 weeks (213.5 days).
You need to benchmark the number of keys per second on your own system to know the time it would take for you. If you can calculate half the keys, it takes twice as long. If your system calculates 10 trillion keys/sec it would take only $\frac1{10}$ the time (10.675 days average, 21.35 days max). I think you get the picture.