Stats haven't always been one of my strong points but there is something I think I can add - something I think a lot of people are overlooking.
Whether or not is would take, on average, X numbers of tries in order to form amino acids - you have to remember that all those different combinations didn't/don't occur one right after the other - they can happen all at once. In other words, evolution doesn't just happen in a straight line - mutations occur everywhere and in every direction. If you look at mutations that have been successful, then it appears that evolution is going in a line (or meandering path).
Just look at how variable HIV is. All these mutations all occuring at the same time - some are viable and some aren't. Some are better at surviving for a while or better at transmission or better at eluding drugs and the body's own systems.
Anyway, hope this made sense. Oh, and one more thing - who ever said that evolution had to be constant? From what I've read there's been some debate as to how constant the rate is.
That's the funny thing about nature - we try to make it fit into very simplistic creations of our own - get very irritated when she refuses to fit :).