First, I've learned on estimating things, take your estimate, double it and add 50% and you will be close in most cases to reality.  As the saying goes in game dev/coding, when you're 90% done, you only have 90% left to go.

Now for your actual question, how soon should you start advertising?  Soon, but you have to be willing and ready to keep updates coming and interact with the community you may develop.  It's a lot of work, but if you expect to get much attention a month before release, you will likely be very disappointed from what I've read.

Depends on your marketing plan. If you're going to spend a million dollars on ads to coincide with release, you probably don't need to do any early marketing  
If you're making a multi-player or high-difficulty game, it may be important to begin gathering a community before you launch, so that on release day there's a group of experienced players already.
If you're not sure of your ability to get attention from press, it might be worth trying to start getting media attention one year early, so that by the time you launch, it's possible to google your game and actually find articles written about it.
If you've got something that is honestly innovative, perhaps you want to keep it under wraps until a month from launch to assure that copycats won't appear until afterwards.

It also depends on what kind of marketing you plan on doing. We ran a blog for our indie game for the first 5 years of development and got pretty much zero press (a single mention in one french YouTube channel), so that's pretty much the same as doing nothing. We just exhibited our game at PAX recently though, and have appeared on several gaming blogs, podcasts and youtube channels, and also recruited a very small community of public beta testers. There's quite a few indie games that do the exhibition and game awards circuits for years before release, and then launch with a bajillion "IGF finalist"/etc logos all over their game.