Being an expert at something is sometimes difficult to discuss, prove, or even validate from your perspective – not to mention from a potential customer's perspective! There are usually two ways to pick an expert: social proof and some seniority—someone recommended by others who has accumulated enough time in the required field. The magic number of spending 10,000 hours flew around for quite some time.
In this video, Derek Muller deconstructs more precise pre-requisites that are much more grounded in science:
The first pre-requisite is that you need to hone your skills with many repeated attempts with feedback (4:54 mark in the video). So no, not "just" 10,000 hours; you also need to be able to have external feedback to help you progress and nudge you to the next stage. If you don't, you will rapidly plateau at a theoretical level of understanding of your field (most management school professors with scarce field experience typically don't accumulate such repeated and direct feedback).
You also need a valid (predictable) environment (6:46 mark). You can play the same slot machine for ten years and won't build expertise (this is a low-predictability environment). In that regard, we could discuss if innovation or entrepreneurship are predictable environments enough to build expertise. I tend to believe so. Not that you can be 100% accurate at it, but you can move out of the way 90% of the possible pitfalls and mistakes. There is tremendous consistency in the way innovation fails!
Another critical element is getting timely feedback (11:20 mark) to connect an action to its performance and build your library of patterns. This largely explains how economists are usually so bad at understanding... the economy. Deconstructing how the 2008 crisis happened years ago doesn't build much expertise.
The last element is to engage in deliberate practice (or pushing beyond your comfort zone – 13:50 mark). That's the skiing rule I learned as a kid: if you don't fall enough, you're not learning. It's probably the most intuitive rule missing from the magic 10,000 hours narrative, but also one we love forgetting about. One of the many ways to build this in our consulting field is to work with many different industries. Working in clinics, where you have one hour to unlock a key issue with a startup you discover in real-time, is also a great challenge.
All that said and done, and while I believe I managed over the years to check all these marks and build pretty decent expertise in innovation, there's always a significant struggle. You see, for many (most?) missions, you know within ten minutes what "should be done." But understanding what could be done and how to bring your customers there... This is a whole different ball game.
One that I increasingly think makes all the difference in the world.