I think one of the traps that libertarian thinkers fall into is the modern assumption that businesses should always be growing and advancing. This question seems to be built upon the premise that your business will always benefit by investing more in it. Most businesses, in my opinion, serve their purpose best when they are small and are able to cater to the specific needs of a few people than trying to mass produce one-size-fits-all products for as many people as possible. Constant growth ends up necessitating massive amounts of waste and other inefficiencies, such as the disposable products that are so commonplace now. When businesses grow too large, it becomes increasingly more difficult for them to adapt to changes in the market, too.
I also think that you are underestimating the value of donations as an investment in your community. By helping people around you when you are doing well, you are improving your situation in other ways, which may or may not be able to be measured in dollars.
I would rephrase your question to this: "When should I shift the primary focus of my investments from myself to other people?"
To that, I would answer, "When you are happy with your situation and you're ready to share." If you're someone for whom that point never comes, people will probably dislike you and resent you, which, ironically, will probably result in circumstances that will detrimentally affect your situation.