Very simple, when your investment in PR gives you positive ROI.
How to do it - you need to learn how to evaluate PR from two aspects -
a. convert all business information into the financial term of costs and benefits and perform a costs and benefits analysis.
b. a major milestone can be achieved or an obstacle to be overcome when applying the PR.
"The goals and methods are always clear and straight forward but the paths/ journeys are always curving"
I do not see further details to your question, so I will respond in a wider manner.
First, If your company or a proeminent member of your company is involved in a scandal (whether personal or business), then you should definitely invest some money in people who know how to reverse the perception produced by the scandal.
Secondly, if you have built an expansion strategy for your company, then you may need to invest in PR in order to equate greater positive perception with the size of your expansion especially if you intend on moving in territories o regions where people have never hear about your services.
Laslty, PR can be a good investment if it will bring to you the 'key connections' or 'the key players' (e.g: political connections, influencers...)who can assist you in entering new markets. Most management consulting firms operate that way in Africa.
Public Relations is a tool in your marketing toolbox to use for creating awareness, attempting to control the public narrative, and solving certain problems. There are all kinds of ways to use PR but you should start with asking yourself what kind of problems you need to solve and then figuring out if PR is the best way to do that, as opposed to assuming you need PR and just waiting for the "right time" to start. It is very hard for a young company to use PR for growth marketing unless there is something truly newsworthy or interesting about the product, company, founders, clients, etc.
Some examples of different ways to apply PR:
You can use press releases to create awareness for the company's launch, a new product or feature announcement, new investment, a new strategic partnership, or anything else you want to announce. Press releases can help you get content out on the web that talks about your features and has backlinks that can drive SEO value. Most press releases will not automatically secure coverage in any good publications unless it involves a large newsworthy company. However, a recent official press release may add a feeling of credibility to your company when potential customers search for you.
Securing press in actual publications is much harder and very rarely has to do with issuing press releases. It requires having or establishing a relationship with journalists or bloggers and adding value to their lives by making it advantageous to them to write about you. That could be as simple as you being very relevant to their audience, giving them a discount for their readers, or building a relationship long before you ever ask for anything by simply giving them inside data or scoops about the industry you're both in. If you can help them write a great article with tons of data from a paid industry survey you conducted, they may return the favor when you have news to announce.
I have used PR about my funding round to attract other investors. We offered a tech publication the news of our funding as an exclusive story. Once they published it, which they would not have if we had used a press release, I used the article to send to other investors.
I have released a press release about a partnership and then emailed the release to clients asking them if they saw our partnership with XYZ company. The brand name of your partner will often increase your credibility, as will the press release, even though you paid to publish it, and it gives you a timely reason to reach out and re-spark a conversation with people in your pipeline.
I have very rarely had a good experience with a PR specialist. There are way better ways to get press if you decide to focus on them, but they involve grassroots outreach usually from the founder.
Feel free to touch base to chat about PR for startups.
When your product or service is offering a solution SO unique that influencers and journalists will want to talk about it at minimum cost.
Part of a successful PR campaign is in the product/service and part is in the storytelling around it - the key for both is to be as single-minded as possible: select 1 benefit, select a VERY specific target group and bring to life your benefit with storytelling.
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