Please imagine after a couple of years you meet someone who didn't know much about a particular skill or in a field such as automobile mechanics or poetry or even cooking etc. His / her record showed you that he shouldn't be able to do any specific task in one of those fields, but in an occasion you find them very specialist in one of these fields. It marvels you and you are going to show your surprise and wonder. Does the self-made sentence below sound natural in this sense:
- Since when have you become such an expert?
If not, please let me know how can you bring it up in a more idiomatic and natural way.