Please keep in mind I am very new to this.
Lets assume solo mining.
To 'dumb it down' to my understanding when you mine bitcoins are you given a mathematical problem to solve, and exactly when you come up with the answer is a bit random.  You could solve the problem within seconds, or you could try every single possible combination, and only hit the correct answer on the last possible 'guess'  This would be the least amount of Mhash's VS the most amount needed.
So here is the question.  Is there a place I can go to find out, and what are currently the maximum amount of hashes required to solve a block?

This thread is not about understanding how any of this stuff works, its to answer the simple question, "If my computer gets an average of 250Mhash/s, what is the maximum amount of time it would take me to solve a block, and get 50 bitcoins?"