If I have to tell you only one thing: it is Distribution.
Most of the entrepreneurs (including me at one point) are so obsessed with the product and adding new features to it. They completely overlooked the fact that Distributing the product is what play single most important role. Even over the quality of the product.

Discovering how to mitigate risk and to increase the probability of success of your business(es).

And you might wish to consider re-wording your question (and therefore to your thinking on this) to "investing his/her time".

Best of luck!

Customer Service
Investor Relations (if not bootstrapping)
Recruiting
Product
Employee Morale / Engagement
Sales
Mentoring (employes)
Being Mentored (trusted advisors)
Physical / Spiritual Health
Relationships (inter-company, personal and industry)

The exact location of time to each is highly dependent on the type and stage of business and what's facing the entrepreneur and their business and life in any moment.

Happy to talk to you if you feel conflicted or confused.

Honestly assess your strengths and focus on those. Partner, outsource, leverage resources and hire in all other areas to bring your product or service to bear.

A mistake many entrepreneurs make is to try to do it all. We may brag we "wear many hats," but what we really do is "roll up our sleeves" as needed then too often get bogged down in areas where we have little or no experience. We tend to get in our own ways by trying to do it all.

We get farther and faster when we leverage the resources and people who can do what we are trying to do more efficiently and effectively than we ever could because it is what they are good at doing.

(Context: I've started and run over half a dozen companies and have seen the most success where I've hired in areas where I am weak and played to my strengths. I've also failed miserably when I tried to do it all or partnered with the wrong people who held on too tightly and inhibited growth).