The interesting thing about investing time is that the distribution is much more uniform versus monetary wealth. Most people probably sleep for around 6-8 hours and work for around 8 hours, giving them another 8-10 hours to either invest or spend. Finances range much more wildly. Some people have negative net worths, so they can't even begin to think about investing, whereas others have billions of dollars. That spread means that ideal financial investment strategies will range wildly. Because we have such similar amounts of time, though, maybe there are some general principles that will be almost universally applicable.
Before we get into those strategies, though, let's talk about what investing time means. I'd define it as devoting time to an activity whose primary benefit won't immediately be realized. School is one example-- you can have a lot of fun in school and benefit immediately from it, but you're really going because you believe that it will pay off when you graduate. I'd say that school is sort of like investing in real estate. You can always use it for something, it's historically been a good investment, but it's a lot of leverae in one big investment, and recently hasn't performed as well.
Spending time with people and building relationships is an investment, not because that process isn't fun, but because the benefits of having strong long term friends are even greater than the immediate pleasure of hanging out with them. This could be analogized to investing in art-- you get to admire it every day, but the value of it increases over time as well.
Going to the gym is in investment. No matter how many presses I do today, I'm not going to be stronger or bigger immediately. But if I continue to go to the gym, I'll become stronger over the long term. A good analogy for this might be a savings account, although I don't think that does justice to the returns of working out. You keep putting something in over a long period of time, and eventually it pays off and continues to appreciate.
Eating healthy is an investment that might be compared to ferreting away a twenty here and there from your paycheck. It can be a tough habit to create, but is only a minor inconvenience on an ongoing basis, and eventually turns into a huge benefit (you know... not dying when you're 75).
Producing work is an investment, just like starting a business. It takes a lot of effort, but produces better results than passive investment.
All learning is an investment. I separate this from school because although you can learn in school, its function is often more to create credentials than to build useful skills. Learning is like buying stocks-- learning any given skill may become very useful in the future or not at all, but by learning a bunch of skills you build a portfolio that will almost certainly be valuable.
These are just some examples off the top of my head. I'm sure that you could come up with a million more. The point isn't to catalog every possible use of time, but rather to illustrate the differences in each vehicle. With those differences in mind, can we come up with some ideas on strategy?
The first thing that comes to mind is that diversification matters. You never really know which investment is going to pay off, so you cover your bases by investing in all different areas. Just as someone could buy one stock and get really lucky (and rich) when it takes off, someone could rest their entire future on school. Maybe they get a perfect job that fulfills them, introduces them to amazing people, and provides for them for the rest of their life. That could happen-- but it's not the most robust strategy.
In traditional investing, there's this idea of rebalancing your portfolio. Every X amount of months or years you reallocate everything to maintain a balance between different asset classes. So if your bonds stay the same, your stocks depreciate, and your real estate appreciates, you sell some of your real estate and use the proceeds to buy more stocks. I won't get into the details here, but this is generally considered to be a good idea.
We could do the same with investing our time. If we spend a year and make a bunch of new friends and date a lot, maybe the next year we can take time away from that and put it where it's needed more. If our health declines one year, it's probably a good idea to refocus on it the next year.
I think it's important to realize, too, that there's such thing as a good investment, a bad investment, and everything in between. A financial investor must look at these things rationally and buy or sell based only on future prospects. If a stocks fundamentals are bad, it's can be correct to sell at a loss. I made this decision when I dropped out of school-- for my specific goals and desired lifestyle, school wasn't going to perform. So even though I had invested time and money into it, I dropped out. On the other side of the coin, I've advised people to quit pickup. For many people it's a great way to expand their social circle and build relationships-- but if you've been doing it for years without making significant progress, it may not be the best investment for you.
Pundits can argue all day long about different stocks, but they all agree on one thing: do invest, start early, and do it consistently. This hold true for investing time. Just as many people never invest money in their life, preferring to blow it on material goods and entertainment, and then find themselves unable to retire, many people spend all of their time rather than invest it, and put themselves into very difficult positions down the road. Each person has the right to come up with their own allocation between investing their time and spending it, but the most important thing is to be aware of the difference, begin investing early, and keep doing it throughout your life.