Blizzard setup the leveling so that you would complete the first three difficulties within the same amount of time that it takes you to reach level 60. The first 31 levels are designed to be spent working your way through Normal difficulty, where you familiarize yourself with bosses and just about every active ability your class has to offer. Levels 31 to 50 you will spend in Nightmare revisiting Act I through IV. Finally the last 19 levels are spent struggling (hopefully) through Hell difficulty. Here are how the levels break down by each boss:

These levels are based on what you should be by the time you reach the bosses. If you don’t use a leveling gem (ruby in a helmet) and go as quickly as possible through everything you might find yourself a few levels behind. You might even find yourself beating Diablo on Nightmare at level 47 or so. If this happens it means you will have to go back and farm Nightmare until you’re level 50, you might as well take your time reaching Diablo so that this doesn’t happen.

Important: Many of the strategies and tips I share here were originally inspired by the people at the Diablo 3 Gold Secrets Forum. If you're serious about making the big bucks in Diablo 3, or just being the best players, you need to signup ASAP.