Malcolm Gladwell taught us in his 2008 book Outliers that it takes ten thousand hours to master any skill.
The World Health Organisation's determination of the average human lifespan and the Gladwell guideline reveal that at the absolute best, a human can master a maximum of 40 skills per lifetime.
Many, of course (Gladwell's critics for instance), will master none. Some with super-human will, super-human lifespans, and the flexibility of the Gladwell guideline will master more than 40 skills. People like The Art of Learning's Josh Waitzkin will reach an extreme form of skill mastery. Others, like Tim Ferriss of The 4-Hour Workweek, will master skills strategically in order to maximise their effectiveness in serving the liver of the lifetime in which they're earned.
But on any skill, the requisite 10,000 hours can't be spent all in one lump-sum. One has no choice but to spread them over years of busy living. Why? Because one also needs to eat and poop and sleep, but also because one's own cognitive limits will block the attainment of those tasty skill gains a lot of the time. That's right: Humans can and do WASTE hours pursuing skills without retaining the value of that time (due to memory limitations, adrenal failure, synaptic fatigue, or even plain old lactic acid) and thereby they lose that time off the skill-counter.
So why bother?
The answer is because without learning new things (like skills), a human doesn't grow. Humans don't, strictly speaking, need to grow. One could probably lap up everything that the lowest rung of Maslow's hierarchy of needs has to offer without so much as growing by a single hour of effort invested in any skill. Growth, in fact, is all the way up on the second highest level of that pyramid. That's high up! You might not even want to climb that high. Hundreds of thousands of humans don't ever trouble themselves to try.
The gravity of that approach, of course, is that it disqualifies one from the possibility of self-actualisation. Contented by the pit stop of the comfort zone, those humans are labelled DNF in the Grand Prix of existence.
So go forth and master some skills. The quality of your future depends on it! So does the quality of your society's future, for without skilled contributors a society becomes destitute.
What will your 40 skills be?