The answer is simple; two part but simple. The first answer is the day you start 
your business. Chances are that if you are reading this, that time has past. If your business hasn't started up yet and you are getting ready to start, congratulations. Starting a business with a well thought out marketing plan will contribute to your success. Its been proven over and over and is essential to success and growth. Spend the necessary time planning, involve employees, work with experts and lay out your road map. Most business owners know they have to market but they have no idea where to start. Consult an expert and spend your time in what you do best.
The second part to the answer to the question of when to plan your marketing is right now. You cant go back in time. There is no time like the present. You've heard it before and its true. Work with an expert, think it through and consider some of these factors:
- Why should someone do business with you?
- Why you and not your competition?
- What are your true benefits that you offer to your prospects and customers (HINT: The benefit you offer that your competition doesn't offer constitutes your competitive advantage).
- How easy are you to do business with?
- What more can you do for your customers?
- How often should you communicate with them and in what form?
- How are you going to let others know about you so they can do business with you (ASK YOURSELF THIS QUESTION HERE: Does everyone that can buy from you, know about you?).
- Do you show current customers the same appreciate and incentives that you show prospects?
- Who do your good customers know (People like to help others and will tell others of good experiences). Capitalize on word of mouth and referrals.
- Humanize your business; put a face on it; People do business with people, not icons, not logos, not brands but people.
These are a few strategic considerations that then go into your marketing plan. Take these considerations and extend them to your message, your target, what you want people to think of when they see your name, logo and brand, how will you get your message to your target market and how often will you do that.
To put the finishing touches on this, think about how you respond to marketing; how you want to be treated; what you want most from those that you buy from. Your prospects and customers feel the same way.
Happy Marketing!!