I purchased a home in the fall of 2012. The reason people say to give yourself X amount of lead time is because they probably didn't have their ducks in a row and weren't ready to buy a house. If you get squared away with the basics before you even start looking at property you can really make an offer and close quite quickly if you find the right house (empty house which doesn't need repairs).
I hired my realtor about two weeks before I made my offer and was about two more weeks from the acceptance of the offer to the closing. It doesn't always go that fast but it can if you do it right. Of course, you might now want it to go fast, depends on your situation.
Things to do before you hire a realtor.
1. Have your financing squared away with your lender so you can focus on shopping.
2. Have you earnest money in cash available so you can make offers.
3. Have your down payment and closing money in cash available so you can close quickly (if you need to).
4. Know what zip code or area you are going to buy in.
5. Know what you want in a house (make a list).
6. Use Realtor.com to search homes in the zip code you are looking. You might even find the house you buy (like I did).
Now, go get a Realtor and let them know about everything you've done. At this point it can go as fast or as slow as you want. Personally, the only thing I need a realtor for is to unlock the door to homes I find on realtor.com so I can look at them, and then file paperwork with the listing agent of the property I am going to make an offier on.