Hi there! I've actually tried this myself a couple times, and read a lot about it. First of all, where do you live? Are your winters cold? If so, you won't be able to permanently transplant the tree outdoors.
When the stem (the part coming out the top of the pit, not the roots) gets four or five inches tall, you can plant the little guy in a pot with good potting soil. Place the potted tree in a sunny window. The tree can be moved outside in warm weather, but like I said, if you have real winters you won't be able to plant the tree in the ground and leave it there year round. If you live somewhere like Florida, go ahead and plant it outside when the tree is about a foot tall.
You might get fruit in four years or so, but I think that depends on what kind of climate the tree is going to "think" it lives in. Your best bet would be to try to trick it into thinking it's in a warm climate year-round. You might have trouble with pollination if you only have one tree.
Good luck with it! You should write a hub about it. I never could get my avocado pits to sprout anything... maybe I didn't clean them properly, or they were bad eggs or something. Anyway, good luck and have fun!