"When should I start marketing my book?"
That question is probably one of the most frequently asked question in the history of author marketing.
If you are asking, you should be marketing.
Do you have social media going? A website? A blog? If not, get on it. A platform is a make or break kind of deal. So and So may have made it big without the above, but so and so is the exception to the rule. You need to become a constant factor in readers eyes before you release so when you do, they care. If they don't care, they typically won't buy.
Marketing isn't something that can come and go. It is a constant aspect of your life as a business. And make no mistake, if you are publishing, you are a business.
So get out there, make yourself known, and remember to always be what you want your readers to see.