To get a sense of how long it would take to find a particular key, consider:
The key has n bits, so there are 2^n possible keys that can be enumerated with those bits.
Each processor can test m keys per second. (I'm assuming each processor has the same performance, and ignoring latency between CPU nodes, I/O latency, or anything else that might slow the system down.)
You have access to p processors.
So the time to process all 2^n keys is:
Note that the value of m doubles once every 18 months (due to Moore's law), so to keep the key finding time constant, you must also add a bit every 18 months. (Adding bits is fairly cheap, but developing faster processors is not!) The value of p is not all that important because p increase linearly as you add more nodes, while n and m increase exponentially. To figure out how long of a key you need for a given algorithm, you simply need to determine the amount of time that you want to keep your data secret for, and choose a number of bits such that (2^n)/p*m is sufficiently large.
I'll let you plug in the numbers and work out the exact times for your favorite system for yourself. :-)