I bill my customers based on value delivered, not on my time exposed to a topic[0].
It could happen that my 15-year tinkering effects in great value delivered.
My point is that time devoted to learning something is not free; you must consider it a business cost.
Regardless of time exposed to the topic at hand, not necessarily a lot of time invested* into something results in a great deal of value to be delivered. You could optimize that time with ultra-learning techniques[1].
The correlation of time exposed to a topic doesn't result in the causation of how much should I charge a customer.
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[0]: https://www.ivanmontilla.com/blog/does-professional-experien...
[1]: https://www.scotthyoung.com/blog/ultralearning/
EDIT: * Improved clarity by adding a word.