When you start your business, the first thing that you do is reach out to people that you already know. This is a no cost method of marketing, you speak to them in person, email or phone and because they already know you, your values and morals, they trust you enough to make an order as soon as they have a need for what you're selling.
As soon as you need to make more money than occasionally selling something to a friend, that's when you need to begin marketing.
Marketing should be an ongoing process. - the ideal plan for your marketing would be the straight lines in this chart. I've been told that ideally 20% of your working time should be assigned to marketing, 60% to the "doing" the job, and 20% to admin. Whereas we're more likely to be the dotted lines that mean we're doing lots of marketing when we have few clients, and then as the number of customers gets more, our marketing falls off.