There are 340282366920938463463374607431768211456 possible 128-bit keys. So if you had a machine that could check a trillion possible keys per second it would take over 10 quintillion years to try all possible 128-bit keys.

The math: https://www.google.com/search?q=2%5E128+%2F+1E12Hz+in+years

I could explain this, but in all seriousness, I think the better answer to this question is to urge you to pop open your Perl, Ruby, or Python prompt and work the math out.

The brute force algorithm is O(2^n)