I would say as soon as you've got a business, and should be part of your marketing plan. Remember PR doesn't need to be a feature in Forbes for example, PR also includes being a guest on a podcast or local radio show, and writing guest blogs / articles on sites that your target market reads. Then as you become more successful you can aim for the bigger and more prominent outlets because you would have this portfolio of publicity under your belt already, that you can show.

You can also register with HARO which has requests for comments and stories on different topics 3 times a day. And if you are in the UK, there is also #journorequests on Twitter where journalists are looking for people's experiences for their stories - it can include stories from business owners and entrepreneurs.

Hope this helps, and if you need help with a plan to do this - I'd be happy to help on a call. Thanks